31O MANUAL OF ZOOLOGY. 



opens into the atrium. The embryo Tunicate is at first gene- 

 rally free, and is mostly shaped like the tadpole of a frog, 

 swimming by means of a long caudal appendage. In one 

 species (Molgula tubulosa) the larval form is destitute of a tail, 

 inactive, and amoeboid, and it almost immediately, attaches 

 itself by means of little outward processes which it develops. 

 Lastly, in several instances the larval caudal appendage has 

 been shown to exhibit a cylindrical rod-like body, which has 

 been paralleled with the chorda dorsalis of Vertebrates. The 

 body in question is a kind of cellular rod, which agrees with 

 the notochord of Vertebrates in giving insertion by its sheath 

 to muscles, and which holds an analogous position to the 

 nervous system. 



Amongst the Salpians a species of alternation of generations 

 has been' observed. A solitary Salpian produces long chains 

 of embryos, which remain organically connected throughout 

 their entire life. Each individual of these associated speci- 

 mens produces solitary young, which are often very unlike their 

 parents, and these again give rise to the aggregated forms. 



The Tunicata are often spoken of as exhibiting thre^ main 

 types of structure, which give origin to as many sections, 

 known respectively as the solitary, the social, and the compound 

 forms. In the " solitary " Tunicaries, the individuals, however 

 produced, remain entirely distinct, or, if not so primitively, 

 they become so. In the "social" Ascidians the organism 

 consists of a number of zooids, produced by gemmation and 

 permanently connected together by a vascular canal, or "stolon," 

 composed of a prolongation of the common tunic, through 

 which the blood circulates. Finally, in the " compound" forms, 

 the zooids become aggregated into a common mass, their tests 

 being fused together, but there being no internal union. The 

 Botrylli, which are familiar examples of the compound Tuni- 

 cates, form semi-transparent masses, often of brilliant colours, 

 attached to various submarine objects, and consisting of 

 numerous zooids arranged in star-shaped groups. They are 

 almost always " very small, soft, irritable, and contractile, 

 changing their form with the slightest movement " (Stark). 



HOMOLOGIES OF THE TUNICATA. The general resemblance 

 between a solitary Ascidian and a single polypide of a Polyzoon 

 is extremely obvious ; each ^consisting of a double-walled sac, 

 containing a freely suspended alimentary canal, with a distinct 

 mouth and anus, and a nervous ganglion placed between the 

 two. The chief feature in the Tunicata, as 'to the exact nature 

 of which there is much difference of opinion, is the branchial 

 or respiratory sac. By Professor Allman this is "believed to be 



