MOLLUSCA. 671 



have only one method of multiplication ; namely — oy means of eggs. To this division belong the genera AiciiU, 



Cynthia, PhaBuiia, Boltenia, with some others. 

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



and which puts forth rpprodaeliet tmdt that develops new Individuals ; whence it results that these »n<»nqi« ,i v , j n 



clusters ur colonies, of which the several individuals an organi- 

 cally united. Each animal lias its uivn heart, respiratory ap- 

 paratus, and system of nutrition; but a eonunou circulation 

 of blood extends through the stem and branches, connecting 

 them all witli each other. The relation between tin- separate 

 aniinals thus bears a strong resemblance to that which sub- 

 sists among the individual polypes of a Serfuloria or o 

 compound polypidom, In whose Btem ami branches a circulation 

 of fluid takes place. To this family belong the genera 

 and 1'orophora. 



3. The Compound Asculians ar.- united in a much more intimate 

 .. manner, a great number of individuals (usually of very minute 



Fro. ;.— forufhoka, a ••>< -iai Awidian, I. tarmehUl size) being grouped together in a single mass, and imbedded in 

 orifice, or month; «, «iuu oi ttij ., btattinml , tino r almost cartilaginous tissue, which has sometimes 



eansl; ', common item* b ° 



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

 tin- individual animals b in.- 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 progn wively Increased. The 

 eggs, on the other hand, give birth to individuals of an entirely new generation ; th( Be 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 which the Ascidian form is gradually evolved ; 

 and from each single individual a cluster may ultimately t> 1 by the process of gemmation. In addition 



to the genera BotryUtu, r I iswm, noticed by Cutler, we have to mention ApHOkm and BtOnymmot 



Bavigny ; Cent eWt m, dmar w fum, and BbtryBofdes of Milne Edwards ; and DUtoma of Saertner. 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 esp -.dally d voted. 



III. The I'yrosomid.-e. as Cuvier has remarked (p. 383), are nearly allied to the BotryUi in the organisation of 

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

 a pile of He star-Shaped clusters of the Botryllus ; and the entire tube is as free as are the hands of . 

 s,l|,:e, instead of being attached like the clusters ofthe Compound Ascidians. Like the Salpss, the I'm,. -una 

 tube acquiri ■ 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 oriflOSS are all directed inwards, ami Open 

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

 lied to issue team the other ; and the continual stream in which it flows I 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 I of rema rk able interest both to the Zoologist and 



to the Naturalist, a few particulars will be a, bbd to what has be. n already stated as to their structure, actions. 



and development The attached species presentus with a most remarkable contrast b twi a the apparent 



Inertness oS their life, and the activity of tie op rations taking place within. If we keep some ,,f the Compound 



Uddiani [which we mayhavi found on a broad-leaved focus cast ashore aft. r a ■.term. | in a vessel of sea-water, 



" we flnd t hem lie there as apathetic as sponges, giv ing few signs of vitality beyond tie- slight pouting-OUt of tube. 



like membranes around apertures which become visible on their surfaces j though 



animation will show as current! Inactive motioninthe water around these apertures, strean and 



whirlpools rushing in ; Indicating that however torpid the creature may externally appear, all the machinery of 



life, the respiratory wheels, and circulatory pumps, are hard at work in its inmost i -." All tb 



operation-, belong, howi-v.-r. to thi Life, and do not indicate any oon* non 



the part of these beings. The currents of water are produced, as alreadj mentioned, bj thi 



clothing tb.- internal membranous surfaces ; and this action we have everj reason to 1 I ade- 



pen. lent i. f the animal's will, and even beyond its Control. It is a curious fact fli.it found 



making their way through tb.- water, after th.s have been deprived of their visceral massbj birds ..■- Banes. 

 The entire nervous system is here reduced to a single ganglionic centn Pig. o), which is situated between tlio 

 two orifices, sends filament- to each of them, and also distributes its branchi ovei the general I the 



mantle. If o organs of special sensation arc perceptible, and the only Indication of common lensib Itj sh..»u 



