As unlikely as it seems, based on embryological evidence, the echinoderms appear to be the most-likely ancestors to the early chordates

Transcription

1 Chordates Members of the Phylum Chordata include animals with which we are probably most likely familiar (including fish, birds, reptiles, amphibians and beasts like ourselves) As unlikely as it seems, based on embryological evidence, the echinoderms appear to be the most-likely ancestors to the early chordates Primitive stemmed echinoderms are thought to have shifted from arm-feeding to filter-feeding acquiring a body plan similar to urochordates Unfortunately, the fossil record is poor and intermediates are lacking. The rest of the evolutionary picture is better documented.

2

3 Chordates Four structural characteristics set chordates apart from all other phyla: a notochord a pharyngeal gill slits Post anal tail a hollow dorsal nerve cord These attributes are always found in the larval forms or early embryo (although they may be absent in the adult).

4

5 Notochord is an elastic, rod-like structure that runs parallel to and between the digestive tract and central nervous system It is found in all chordate embryos and serves as a support structure Axis for muscle attachment The notochord is composed of fluid-filled cells wrapped in a fibrous sheath

6 Notochord In animals like ourselves, bony structures called vertebrae develop near the notochord and eventually replace it during embryogenesis In adult humans, and other mammals, the notochord is preserved as the nucleus pulposus, a small gel-like region at the core of each of the intervertebral discs between adjacent vertebrae.

7 Notochord The notochord resists shortening in length, but is flexible laterally. Without a notochord, lateral muscle contraction collapses the body along its length. With a notochord muscle contractions on alternating sides efficiently flex the body in simple swimming strokes

8 Dorsal, hollow nerve cord forms the central nervous system (brain and spinal cord) of all chordates The chordate nerve cord is positioned dorsal to the gut tube, while the nerve cord of most non-chordates is found ventral to the digestive tract. The chordate nerve cord is also hollow and fluidfilled, while that of non-chordates is typically solid

9 Dorsal, hollow nerve cord the chordate nerve cord forms by a invagination, an embryonic process in which neural tissue gathers dorsally on the outer surface of the embryo, folds into a tube, and then sinks inward (invaginates) to take up its internal position The nerve cords of non-chordates do not form by invagination

10 Dorsal, hollow nerve cord

11 Pharyngeal slits Pharyngeal gill slits are cuts in the pharynx that connect to a cavity surrounding the pharynx For organisms in which they remain in the adult, they are often elaborated into respiratory structures (and are sometimes involved in filtering food during feeding). The morphological equivalent of gill silts are seen briefly during our own development (weeks 4-5), but they usually close or develop into other structures

12 Pharyngeal slits In ancestral chordates and modern urochordates and cephalochordates pharyngeal slits function as suspensio-feeding structures. Vertebrates have modified the slits and their support structures for other functions including gas exchange, jaw support, and hearing.

13 Post-anal tail Chordates follow the anus with a tail of variable length is a muscular region of the body that extends beyond the anus. In us, the tail is short and fused (the coccyx at the base of your spine) The chordate post-anal tail includes skeletal support and musculature that improves the locomotion of many aquatic chordate species.

14 Chordata The phylum Chordata includes three subphyla: Urochordata (tunicates or sea squirts) Cephalochordata (lancelets) Vertebrata (vertebrates)

15 Subphylum Urochordata

16 Subphylum Urochordata At first glance, could be mistaken for a sponge Adult tunicates look like small sacs (about 3 cm tall) and are stationary, lacking a nerve cord, a notochord, and a post-anal tail However, tunicates begin life in a larval state, which have a post-anal tail, a nerve cord, and a notochord

17

18 Subphylum Urochordata Urochordates have a notochord that extends from just behind the tail to the head (rather than from head to tail; Urochordata means "tail-cord") Many urochordates are more commonly referred to as sea squirts Contain cancer-fighting chemicals

19 Subphylum Urochordata The body of an adult tunicate is quite simple being essentially a sack with large gill structures that form two siphons through which water enters and exits. Water is filtered inside the sack-shaped body.

20 Subphylum Urochordata

21 Subphylum Cephalochordata Lancelets are common bottom-dwelling forms that possess all four chordate characteristics They dig into the sand and lie with their anterior end protruding from the burrow Unlike the urochordates, the notochord extends along the entire length of their body This structure imparts rigidity to their body and permits more coordinated swimming movements

22 Cephalochordata

23 Cephalochordata

24 Subphylum Vertebrata notochord replaced by a vertebral column composed of bone and/or cartilage The vertebral column, along with the cranium, limb girdles, and limbs, make up the endoskeleton This internal skeleton is an adaptation for efficient locomotion, as was the notochord

25 Subphylum Vertebrata Subphylum Vertebrata has several divisions you need to be familiar with Superclass Agnatha Jawless Fish; Lampry Eel; Ostracoderm (fossil)

26 Superclass Agnatha Many species are parasitic (they attach to the outer surface of a fish with their sucker-like mouth) Rasping teeth (arranged in a whorl) cut into the host The lamprey then sucks blood from the wound When it is finished its blood meal, the fish is released. The injured fish usually dies from blood loss or infection

27 Superclass Agnatha

28 Superclass Agnatha The agnaths are considered to be an evolutionary dead end, a side branch in the phyletic tree that did not lead to the more advanced fish The first agnaths were ostracoderms Though extinct, they give us clues into how vertebrates evolved from the cephalochordates

29

30 Adult Larva