MOLLUSCOIDA. 
311 
The expansions still represent microscopic bee-hives, the inhabitants 
°f which enjoy at once a common and an independent existence. As 
it is with the polypiers, so it is here ; each eats for the benefit of itself 
a nd for the community. Labour and nutrition for the community, 
labour and food for itself. 
Probably there l'eign among the inhabitants of one of these groups 
sentiments of brotherhood of which we have no idea. Since that 
"hieh is digested by one of the family is beneficial, up to a certain 
b°int, to all, ought there not to be between the diverse individuals, 
especially between those nearest to each other, a physiological bond, 
niore or less direct, to draw them together— a moral bond, more or 
less strong? What strange vitalities, organic combination, to find 
^nder the brilliant blue of the ocean ! 
Tunicata. 
On seeing one of the Tunicata for the first time, a stranger to 
^Oology 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 
*ith marine plants and poly piers. They have neither arms, nor feet, 
tl °r head. But then they have a mouth, placed at the entrance ot a 
digestive tube, and, in connection with the latter, a special opening 
^tended for evacuations. The mouth is preceded by a great cavity, 
10; walls of which are covered with vessels ; for this cavity is the seat 
°1' respiration, and is covered with vibratile cilia. Thus the same 
Ca nal serves first for respiration, and then, farther on, for digestion . 
pother instance of the economy of Nature. Another remarkable 
^stance of circulation is found : they have a heart, but no head. 
This heart is the centre of a well-developed vascular system, but 
Ver y unlike what usually obtains. The blood which traverses it takes 
Sll °b a course, that, in the space of a very few minutes, the heart 
c bang es it s aurical into ventrical and its ventrical into aurical blood, 
ft the same time the arteries are changed into veins and the veins 
hdo 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, 
t is an unique ganglion, connected with divers small fillets. The 
