138 



ZOOLOGY. 



There are in Salpa two kinds of individuals, i.e., the 

 solitary and the aggregated or chain Salpae. 



The young of many Ascidians are horn with a tadpole- 

 shape, in which there is a notocord, which, however, does 

 not extend to the brain, and the mouth-opening is dorsal 

 rather than ventral (Fig. 180), otherwise the larval Ascid- 

 ian is strikingly like the embryo lamprey: in both, the 



ne **d jf/J 



'Jit sp 

 Fio. 179. — Diagram of embryo Lamprey. 



Fig. 180.— Diagram of larval Ascidian. Lettering as in Fig. 179. m, mouth; t, 

 digestive tract; sp, spiracles in the pharyngeal portion; ht, heart; e, eye; er, ear; 

 br brain; nc, nervous cord : b', b", mid brain; <7, cerebellum; spv, spinal nerves: 

 n, notocord; ol, nasal cavity; s, suckers (.their homologues also occur in young 

 gar-pikes and tadpoles). 



mouth leads into a pharynx with gill-openings; both have 

 a rudimentary brain, and a notocord situated beneath the 

 nervous cord. The young Ascidian, then, is seen to have 

 the fundamental characters ascribed to the vertebrates, 

 though it loses them before growing up. 



Class II. — Leptocardii (Lnncelet). 

 The lancelet is the only type of this class. The body is 

 four or five centimetres in length, slender, compressed, 

 pointed at each end. The muscular segments are distinct 

 to the naked eye. From the mouth to the vent is a deep 

 ventral furrow, and a slight fin extends along the back and 

 centrally as far front as the vent, 



