II. TUNICATA 



441 



or arches. The ventral trunk begins as a subintestinal vein under the 

 intestine, branches as a portal vein over the liver and, reuniting again in 

 a ventral vessel, and joined by the paired veins (jugulars, cardinals and 

 Cuvierian ducts) continues forward, as the aorta ascendens, below the 

 gills. From this the gill arteries pass up between the gill slits and form 

 the dorsal vessel, the aorta de- 

 scendens. A true heart is lacking, 

 but various parts of the vessels a 

 part of the ventral trunk and the 

 bases of the gill arteries are con- 

 tractile, whence the name Lepto- 

 cardii. 



The digestive portion of the 

 tract lies in a true ccelom, which 

 extends forward (fig. 494, co) into 

 the gill-walls (branchial ccelom) and 

 into the outer walls of the peri- 

 branchial chamber (peribranchial 

 ccelom). In a distinct part of the 

 peribranchial ccelom are the gonads 

 (g), a series of pouch-like cell folli- 

 cles which allow their products to escape into the peribranchial chamber. 

 Into this chamber also empty the excretory organs (n), a series, on right 

 and left sides, of ciliated canals. Each canal begins with at least one 

 ciliated nephrostome in the ccelom and opens separately and has the 

 characteristic solenocytes like an annelid nephridium (fig. 495). 



Like the structure, the development is comparatively simple. The following 

 points deserve special mention: (i) The eggs have a nearly equal segmentation 

 (fig. 101). (2) A typical invaginate gastrula (fig. 107) occurs. (3) The meso- 

 derm arises as a series of pouches, right and left, from the mesenteron, which 

 later separate and form the primitive segments. Hence these are clearly 

 mesothelial in nature. From the cavities of these arises the body cavity, which 

 is consequently an enterocoele. (4) The dorsal surface of the entoderm between 

 these coelomic pouches separates from the rest and forms the notochord, which 

 lies between the digestive tract and the nervous system. (=;) The nervous 

 system arises from a longitudinal groove which becomes folded into a tube and 

 is connected for a while with the digestive tract by a neurenteric canal. 



Amphioxus* Asymmetron,* Hcteroplrnron. The animals bury themselves 

 in the sand, with only the mouth above the surface. 



SUB PHYLUM II. TUNICATA (UROCHORDA) . 



The adult Tunicata, or sea-squirts, bear some resemblance to the 

 clams in the possession of a mantle and incurrent and excurrent orifices, 

 usually close together. Hence they were long called molluscs; later they 



FIG. 495. Excretory tubules of Am- 

 phioxus (after Boveri and Goodrich), 

 i, a whole canal with several nephro- 

 stomes and the connected bunches of 

 solenocytes. A", upper end of gill cleft; 

 P, opening of canal into peribranchial 

 chamber. 2, a bit of canal wall with 

 several solenocytes. 



