382 



Tl'nicata.- 



-5I0LLUSCA.- 



-ASCIDIANS. 



moriihoses. Tlie larvoe are like tadpoles in shape, are 

 solitary and free, swimming rapidly by the aid of the 

 iiiidulatory movements of a long tail. They attach 

 themselves occasionally; they then change their form, 

 lose their tail, and after a few days' free and locomotive 

 life, finally fix tliemselves. In addition to reproduction 

 by ova the compound Tunicaries have been ascertiiined 

 to possess a second mode, viz. — by gemmation. This 

 process is superinduced upon the young Ascidian after 

 its development from the ovum. " The individuals 

 formed by the gemmation of the primary bud of the 

 young Ascidian are, instead of being detached, retained; 

 the process of gemmation being regulated so as to pro- 

 duce the characteristic pattern in which the different 

 individuals are grouped in the mature compound ani- 

 mal. " — (Owen.) The greater number of the Tunicaries 

 are fixed in their adidt state; but there are others that 

 are always free, and only found in the open ocean. 

 They thug form two natural groups. Of the first or 

 fixed group, some of the individuals become entirely 

 distinct, in others they remain connected by a vascular 

 canal, and in a tliird set all the individuals become 

 blended into a common mass. These groups form the 

 Snlilary, the Social, and tlje Compound ascidians of 

 authors. The animals belonging to the class Tunkata 

 are all marine, and are numerous in all seas, being 

 found from low water to a considerable depth. " Ad- 

 hering to rocks and sea-weeds," says Mr. Adams, 

 " these strange, bag-shaped, leathery bodies may be 

 seen along tlie strand at low water, ejecting when 

 touched the sea-water to some distance; and on the 

 ocean their lengthened sinuous chains, or pellucid 

 pliosphorescent tubes, cannot fail to arrest the eye of the 

 voyager. The compound forms exhibit, in the varied 

 airangement of the individuals composing the general 

 mass, a lunuber of elegant stems and flowers of curious 

 and complicated design. Towards the northern shores 



they are sombre in their colours, but in the sunny 

 regions of the south their hues assume the brightest 

 dj'cs, and vie with those of the corallines and actiniie 

 that people the bed of the ocean. It was in the 

 Ascidian Tunicaries that MM. Audouin and Milne 

 Edwards first discovered the metamorphoses of the 

 Molhisca, and their discoveries have since been ex- 

 tended by the laborious researclies of Sars and Loven. 

 It was among these singular beings that Van Hasselt 

 discovered " a heart of such extraordinary character, 

 changing incessantly its auricle to ventricle, and its 

 ventricle to auricle — its arteries to veins, and its veins 

 to arteries." Among the Salpian Tunicaries it was, 

 moreover, that Chamisso made the no less extraordinary 

 discovery that a 'Salpa mother is not like its daughter 

 or its own mother, but resembles its sister, its grand- 

 daughter, and its grandmother.' The Pyrosomes afford 

 a spectacle to the naturalist of unexampled beauty 

 when, seen by myriads from the vessel in the niglit, 

 they gleam with phosphorescent radiance, forming vast 

 shoals of mimic pillars of fire, illuminating all around 

 with a green unearthly glare. The most curious fea- 

 ture in the history of these soft-shelled niolhisks 

 is the fact that many among them form communities 

 of beings, like the corals, a commonwealth of beings 

 bound together by common and vital ties Each star is 

 a family, each group of stars a community — individuala 

 are linked together in systems, systems combined into 

 masses." 



Tlie Class Tunicata has been divided into two orders 

 or divisions by Dr. Fleming, according as the inner or 

 muscular tunic or sac is more or less detached, or is 

 adherent to the external tunic. Those individuals 

 which have this inner sac more or less detached from 

 the external sac are termed Dichitonida ; and those 

 which have it adhering throughout to the external tunic 

 are termed Moiwchitonida, 



Order— DICHITONIDA. 



Inner sac more or less detached from the external 

 tunic ; united to it at the two orifices. Branchiae large, 

 equal, spread on the inner surface of this sac. Bran- 

 cliial orifice with an inner membranaceous ring and 

 circle of tentacles. 



This order contains four families, three of which 

 are fixed, and the fourth floating. 



Family I.— ASCIDIID.iE {Simple Ascidians). 



This family has been thus characterized: — Animal 

 simple, fixed, solitary or gregarious; not united into 

 groups by a common integument ; oviparous ; not 

 gommiparous ; sexes united ; branchial sac simple 

 or disposed in (eight to eighteen) deep and regular 

 folds. 



The species of Ascidians are very abundant in the 

 northern seas, on the British coasts, the northern shores 

 of Europe, and in the Mediterranean. "Barely is the 

 dredge drawn up from any sea-bed at all prolific in 

 submarine creatures," says Professor Forbes, "without 



containing few or many irregularly-shaped leathery 

 bodies, fixed to sea-weed, rock, or shell by one extremity 

 or by one side, free at the other, and presenting two 

 more or less prominent orifices, from which on the 

 slightest pressure the sea-water is ejected with great 

 force. On the sea-shore, when the tide is out, we find 

 similar bodies attached to the under surface of rougli 

 stones. They are variously, often splendidly coloured, 

 but otherwise they are unattractive or even repulsive 

 in aspect. These creatures are AscidiiV, properly so 

 called. Numbers of them are found clustering among 

 tangles, like branches of some strange semitransparent 

 fruit. They are very apathetic and inactive, living 

 upon microscopic creatures drawn in wit'i currents of 

 water by means of their ciliated respiratory oi'gans. 

 The leathery case is often incrusted with stones and 

 shells, decorated with parasitical though ornamental 

 plumes of corallines, and not seldom perforated by 

 bivalves which lodge themselves snugly in the tough 

 but smooth skin." Some of the Ascidians appear to 

 have been well known to Aristotle ; and his description 



