396 Comparative Morphology of Chordates 



Fig. 305. Balanoghssus embryos. (A) Horizontal section of a young embryo, 

 showing the origin of mesodermal pouches. McBride and others have noted the 

 similarity of this section to that of a young Amphioxus embryo as evidence of the 

 close affinity of the two animals. (B) A young Balanoghssus larva (tornaria) with 

 five pairs of gill-slits, viewed from the left side. The gill-slits of Balanoghssus 

 resemble those of Amphioxus. On the other hand, the young larva of Balanoghssus 

 is strikingly like the larva of echinoderms. (After Bateson. Courtesy, Neal and 

 Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



The larva of Balanoglossus, known as a "tornaria" (Fig. 305), is 

 very similar to the larva of an eehinoderm. As in echinoderms, the 

 blastopore becomes the anus. The subphylum, therefore, is included in 

 the Deuterostomia. 



Subphylum Urochorda 



The urochordates or tunicates are all marine. Most of them, in the 

 adult stage, are saclike in form and are sessile ("sea-squirts"), attach- 

 ing to submerged objects such as rocks or wharf pilings in shallow or 

 moderately deep water. Some sessile forms produce colonies of numer- 

 ous individuals attached to one another. In some of these colonial forms 

 the individual is very minute — a small fraction of an inch in diameter — 

 but some of the isolated sea-squirts may be several inches long. A few 

 of the tunicates are free-swimming in the adult stage. Most tunicates 

 are hermaphroditic. In some both sexual and asexual reproduction oc- 

 curs. About 1400 species have been recognized. 



The most significant thing about the tunicates is the fact that, in 

 the early stages of their embryonic development, they closely resemble 

 vertebrates, and in nearly all of them the embryo develops into a larval 

 form which is definitely chordate in structure (Fig. 306). It has a typ- 

 ical notoehord extending from the rear of the trunk to the tip of the 

 long I ail. Dorsal to the notoehord is a tubular nerve-cord whose en- 

 larged anterior end is suggestive of a brain. This "brain" contains an 

 eyelike ocellus and a "static organ," perhaps earlike. The anterior 

 region of the digestive tube is expanded into a pharynx whose wall is 

 perforated by several gill-slits (stigmas) which communicate in- 

 directly with the exterior by opening into a peribranchial or atrial 

 cavity which opens to the exterior. But there is no segmentation of the 



