MOLLUSCA. 



671 



Fig. 7.- 

 orifice, or 

 canal; t,co 



have only one method of multipUcation ; namely-oy means of eggs. To this division belong the genera Asddia 

 Oynthia, PhaUusia, Boltenia, with some others. ' 



2. The Social Ascidians adhere to soUd bodies by a sort of root or creeping stem, which runs along their surface, 

 and which puts forth reproduetive buds that develope new individuals ; whence it results that these animals live in 

 ^ " clusters or colonies, of which the several individuals are organi- 



cally united. Each animal has its own heart, respiratory ap- 

 paratus, and system of nutrition ; but a common circulation 

 of blood extends through the stem and branches, connecting 

 them all with each other. The relation between the separate 

 animals thus bears a strong resemblance to that which sub- 

 sists among the individual polypes of a SertvXaria or other 

 compound polypidom, in whose stem and branches a circulation 

 of fluid takes place. To this family belong the genera Cla-odl\i\a 

 and Forophora. 



3. The Compound Ascidians are united in a much more intimate 



manner, a great number of individuals {usually of very minute 



iBo^HoKA, a locial Ascidian I, branchial gize) being grouped together in a single mass, and imbedded in 



uih ; a, analoriDce; f, scomacn; i, intestinal . o » ««v^* »*• 



on stem. a gelatmous or almost cartilaginous tissue, which has sometimes 



a very firm and even leathery integument, that serves as a mantle common to the entire cluster ; the covering of 

 the individual animals being a very delicate membrane. On the surface of the mass are a number of small 

 orifices, which have usually the form of six-rayed stars ; these are, some the oral or branchial, and others the 

 anal openings, of the individuals imprisoned in the mass. Sometimes, however, the anal orifices of all the indi- 

 viduals are united into a common cloaca. The propagation of these singular beings, which possess a high or- 

 ganization, although looking like masses of inert jelly, takes place like that of the preceding family, in two dis- 

 tinct modes — namely, by gemmation or budding, and by the sexual production of ova. The reproductive buds are 

 formed in the connecting tissue, and thus the number of individuals in a cluster is progressively increased. The 

 eggs, on the other hand, give birth to individuals of an entirely new generation ; these in their young state have a 

 form very different from that of the adults, and have an active power of locomotion, by which they are carried 

 to a distance from the parent stock, to establish a new colony at a distance. Having become attached to some 

 fixed body, they begin to undergo a series of metamorphoses, by wliich the Ascidian form is gradually evolved ; 

 and from each single individual a cluster may ultimately be generated by the process of gemmation. In addition 

 to the genera Botryllus, PolycUnum, and Synoicum, noticed by Cuvier, we have to mention ApUdmm and Sidnyum of 

 Savigny ; Leptoclinum, Amaroitcium, and BotryUoides of Milne Edwards ; and Distoma of Gaertner. It is to the ex- 

 position of the structure and relations of this most interesting group, that the admirable memoir of Professor 

 Milne Edwards, in the Eighteenth Volume of the " Memoirs of the Institute of France," is especially devoted. 

 III. The Pykosomid-E, as Cuvier has remarked (p. 383), are nearly allied to the Botrylli in the organization of 

 the individual animals, but the individuals are united into tubes, each of which may be said to consist of 

 a pile of the star-shaped clusters of the Botryllus; and the entire tube is as free as are the bands of aggregate 

 Salpae, instead of being attached like the clusters of the Compound Ascidians. Like the Salpae, the Pyrosoma 

 tube acquires a progressive motion through the water from its respiratory current ; for whilst the branchial or 

 oral orifices of the animals all lie on the outside of the tube, the anal orifices are all directed inwards, and open 

 into the central channel, which is common to all. This central channel being closed at one end, the water is 

 compelled to issue from the other ; and the continual stream in which it flows occasions a movement of the mass 

 in the opposite direction. 



The attention which has been given of late to this tribe of animals, has caused a great increase of our know, 

 ledge as to their structure and actions ; and as the group is one of remarkable interest both to the Zoologist and 

 to the Naturalist, a few particulars will be added to what has been already stated as to their structure, actions, 

 and development. The attached species present us with a most remarkable contrast between the apparent 

 inertness of their life, and the activity of the operations taking place within. If we keep some of the Compound 

 Ascidians (which we may have found on a broad-leaved fucus cast ashore after a storm,) in a vessel of sea-water, 

 "we find them lie there as apathetic as sponges, giving few signs of vitality beyond the slight pouting-out of tube- 

 like membranes around apertures which become visible on their surfaces ; though a closer and microscopic ex- 

 amination will show us currents in active motion in the water around these apertures, streams ejected, and 

 whirlpools rushing in ; indicating that however torpid the creature m.iy externally appear, all the machinery of 

 life, the respiratory wheels, and circulatory pumps, are hard at work in its inmost recesses." All these active 

 operations belong, however, to the vegetative life, and do not indicate any consciousness or voluntary exertion on 

 the part of these beings. The currents of water are produced, as already mentioned, by the agency of the cilia 

 clothing the internal membranous surfaces ; and this action we have every reason to believe to be quite inde- 

 pendent of the animal's will, and even beyond its control. It is a curious fact that Salpae are sometimes found 

 making their way through the water, after they have been deprived of their visceral mass by birds or fishes. 

 The entire nervous system is here reduced to a single ganglionic centre (Fig. 8, c), which is situated between the 

 two orifices, sends filaments to each of them, and also distributes its branches over the general surface of the 

 mantle. No organs of special sensation are perceptible, and the only indication of common sensibility shown 



