PROTECTIVE FEATURES IN YOUNG OF VERTEBRATES. 115 



complex array of larval spines along the trunk, are typical illustra- 

 tions. The young sharks, dogfishes, and rays are too familiar 

 to demand more than a passing notice. Those not born alive 

 — and many selachians are viviparous — are safely packed in a 

 dense horny case secreted by a special gland in the oviduct, and 

 comparable to the calcareous shell of the fowl's egg. This squarish 

 egg-case is lined by a silky membrane enclosing abundant fluid, 

 in which the young fish, attached to a large ball of yolk, floats 

 securely. There is no amnion, such as reptiles, birds, and mam- 

 mals possess, nor do large epidermal spaces develop beneath the 

 larval skin, as in many bony fishes and cyclostomes, for these 

 cartilaginous fishes remain during a long period within the egg- 

 case, and are robust and even predaceous immediately on hatching 

 out. Like young partridges they are well able to look after them- 

 selves at the moment of leaving the egg and entering upon their 

 independent life in the outer world. 



To speak of the invertebrates is beyond the scope of 

 this paper, but reference must be made to one group of 

 creatures, long classed as near relatives of the mollusca, viz., 

 the tunicates, popularly known as the ascidians or sea-squirts, 

 but now grouped by the more exact zoology of to-day 

 in close intimacy with the Vcrtebrata. Amongst the solitary 

 ascidians, Appendicularia (also called Oikopleura) is interesting, 

 not only from the fact that certain vertebrate features are especi- 

 ally well-marked in it, but from a transient larval structure which 

 it possesses, and which recalls the layers of membrane and fluid 

 briefly described in the foregoing remarks. From the eggs of 

 Appendicularia a strange little tailed creature like a wriggling gnat 

 emerges. It is of glassy transparency, and undulates actively 

 through the surface waters of the sea by the vigorous movements 

 of its long blade-like tail. It possesses, as close examination 

 reveals, an oval body with mouth, gill-slits, eye, ear, heart, and 

 rod-like backbone — features which entitle it to rank high in the 

 scale of animal life. An animal so perfectly organised would be 

 in constant peril in the open sea, and the larva secretes, probably 

 from the integument, a loose mass of clear jelly, which completely 

 envelopes the body and leaves merely the muscular tail free. In 

 this translucent blanket, usually called the "house" of the 

 ascidian larva, it is protected from many dangers, though it pays 



