MOLLUSCOiDA. 309 



it is with the corals, so it is here ; each eats for the benefit of itself 

 and for the community. Labour and nutrition for the community, 

 labour and food for itself. 



TUNICATA. 



On seeing one of the Tunicata for the first time, a stranger to 

 zoology would scarcely take them for animals at all. Almost always 

 attached to submarine rocks, these beings have the form of a simple 

 sac. Their skin, gelatinous, horny, or rock-like, is at times covered 

 with marine plants and polyps. They have neither arms, nor feet, 

 nor head. But then they have a mouth, placed at the entrance of a 

 digestive tube, and, in connection with the latter, a special opening 

 intended for evacuations. The mouth is preceded by a great cavity, 

 the walls of which are covered with vessels ; for this cavity is the seat 

 of respiration, and is covered with vibratile cilia. Thus the same 

 canal serves first for respiration, and then, farther on, for digestion : 

 another instance of the economy of Nature. Another remarkable 

 instance of circulation is found : they have a heart, but no head. 



This heart is the centre of a well-developed vascular system, but 

 very unlike what usually obtains. The blood which traverses it takes 

 such a course, that, in the space of a very few minutes, the heart 

 changes its aurical into ventrical and its ventrical into aurical blood. 

 At the same time the arteries are changed into veins and the veins 

 into arteries. The consequence is, that the current which traverses 

 these canals changes its direction with each contraction of the heart. 



Simple as is their organization, the Tunicata have a nervous system. 

 It is an unique ganglion, connected with divers small fillets. The 

 organs of sensation present themselves in a very rudimentary fashion. 

 We find eyes, and, after very minute search, a single ear has 

 been found. They are propagated by budding, and also from eggs. 

 The young are subject to some very curious metamorphoses, some 

 particulars of which will be given farther on. 



The Tunicata are divided into Ascidia and Salpa, to which some 

 naturalists add the Bracliiopoda. 



ASCIDIANS. 



The Ascidia, from the Greek word aa^iov t leather bottle, have, as 

 the name indicates, the shape of a bottle or purse. The analogy becomes 



