386 MANUAL OF ZOOLOGY. 



central homogeneous, structureless, and elastic rod, surrounded by a 

 sheath of large nucleated cells, and the former is not primitively composed 

 of cells. (Reichert.) 



6. The perforated branchial sac is not a dilated pharynx, but appears to 

 correspond rather with the branchial chamber of the Lamellibranchiata, 



From the above, therefore, the general conclusion may safely be drawn, 

 that we are not at present in a position to give a dogmatic account as to 

 even the most important phenomena in the development of the Tunicates. 

 We may further conclude that the view that the development of the Tuni- 

 cates is fundamentally identical with that of the Vertebrata is as yet not to 

 be regarded as scientifically proved. 



Amongst the Salpians a species of alternation of genera- 

 tions has been observed. A solitary Salpian produces long 

 chains of embryos, which remain organically connected 

 throughout their entire life. Each individual of these asso- 

 ciated specimens produces solitary young, which are often very 

 unlike their parents, and these again give rise to the aggre- 

 gated forms.* 



The Tunicata are often spoken of as exhibiting three 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 (fig. 200, C), the indi- 

 viduals, 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, each like a soli- 

 tary Ascidian, produced by gemmation and permanently con- 

 nected 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 (fig. 200, B) 

 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). 

 The atrial apertures of all the zooids of each stellate system 

 open into a common central cloaca (fig. 200, B.) 



HOMOLOGIES OF THE TuNiCATA. The general resemblance between 





* These cases have been, however, otherwise explained, and asserted to 

 be an abnormal mode of sexual reproduction, the solitary and chained in- 

 dividuals not being the offspring of each other, but being the older and 

 younger progeny of the same parent. 



