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 born 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 
Fig. 179.— Diagram of embryo Lamprey. 
Fig. 180.— Diagram of larval Ascidian. Lettering as in Fig. 179. m, mouth; 
digestive tract; sp, spiracles in the pharyngeal portion; ht^ heart; e, eye; er, ear; 
6r, brain; ?ic, nervous cord; h\ b'\ mid brain; c/, cerebellum; sjjii, 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 (Lancelet). 
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 aji(J 
ventrally as far front as the vent, 
