672 



MOLLUSCA. 



d 



by these animals, is the contraction of the mantle when the surface is touched, or when some irritating 

 particle is drawn into the branchial orifice ; by this contraction a jet of water is spurted out, sometimes 

 to a considerable distance. No beings possessed of a complex internal structure, a 

 distinct stomach and alimentary tube, a pulsating heart, and ramifying vascular 

 apparatus, with branchial appendages for aerating the blood, and highly-developed 

 secretory and reproductive organs, can be imagined to spend the period of their ex- 

 istence in a manner more completely vegetative than these 



All the Tunicata above described appear to participate in a very remarkable peculi- 

 arity in the function of circulation. The heart is very simple in its structure, being 

 merely a contractile dilatation of the principal trunk, without any distinct division into 

 an auricle and ventricle, or a receiving and impelling cavity. This trunk first supplies 

 blood, as in other Mollusca, to the mass of viscera, from which the fluid proceeds on- 

 wards to the respiratory surface, to be distributed over this for aeration ; another por- 

 tion, however, in some Ascidians, is sent direct to that surface. The whole fluid, after 

 being thus exposed to the oxygenating influence of the streams of water continually 

 passing over the branchial membrane, is collected by vessels which unite into a single 

 trunk that conveys it back to the heart. This may be said to be the direct course of the 

 or asc'idi'a • o, branchial or'i- circulation, because it is that which takes place in the Mollusca generally. But it is 



fice or mouth;*, vent; c, gan- not constant in any of the Tunicata. After the heart has constituted its pulsations 



glmnjrf, mantle (the external * 



coat being removed). for a time, so as to propel the blood which it has received from the gills through the 



systemic trunk, its action becomes feebler, and the movement of the blood slower ; a slight pause then 

 occurs ; and the pulsations then recommence, but in the opposite direction. The end of the heart which is 

 connected with the systemic trunk now begins to contract first, and propels the blood towards the other 

 extremity, into which opens the channel that previously brought it back from the gills ; through this channel 

 it now passes to the branchial surface ; and thence it returns to the heart through the vessels which distribute 

 it to the viscera. After continuing in this reversed direction for a time, the circulation again returns to its 

 original course. The period of alternation varies considerably even in the same individual ; from thirty 

 seconds to two minutes intervening between every change. The average time of the flow in each direction 

 is, however, the same. In the Social Ascidians, the circulating apparatus of all the individuals of a cluster 

 is connected by trunks passing along the stem and branches (Fig. 7). The trunk that carries back the 

 blood from the branchial surface does not at once proceed to the heart, but enters the footstalk, and joins the 

 main trunk contained within the stem ; and it is a branch proceeding from this trunk, and passing along the 

 footstalk, that enters the heart. The alternation of the circulation takes place in these as in the solitary spe- 

 cies ; and it is curious that, if the flow of blood through the footstalk of any individual be prevented by a liga- 

 ture, the circulation then takes place after the manner of the solitary species, — the blood being returned at 

 once from the branchial surface to the heart, or being propelled directly from the heart to that surface. 



The metamorphosis undergone by the simple and compound Ascidians is not one of the least curious parts of 

 their history. They do not begin life as fixed animals ; but as independent, free-moving, tadpole-like embryos. 

 The larva, as it appears in the egg, is at first an oval disk ; a sort of tail is then formed, by a prolongation ofa portion 

 of this disk, round which, however, it is at first wrapped ; arm-like projections spring from the head, which may 

 then be likened to a hydroid zoophyte ; and in this condition it comes forth from the egg, and swims freely 

 through the water by means of its rapidly- vibrating tail. It then becomes attached by its arms to rocks or sea- 

 weeds ; the tail disappears ; that which was the head now becomes the base, sending out root-like projections by 

 which it is firmly held ; the visceral mass is gradually developed in the neighbourhood of this ; the two orifices 

 are formed at the opposite extremity ; and the final form of an Ascidian begins to be manifested. The production 

 of the cluster, in the compound species, by gemmation from the first individual, takes place subsequently to the 

 full development of the latter ; and the buds usually proceed from the root-like processes which it has sent forth. 

 One other remarkable fact concerning these curious animals deserves special mention. It has been lately dis- 

 covered that the gelatinous mass in which the individuals of the compound Ascidians are imbedded, consists al- 

 most entirely of a substance cellulose, which has been usually regarded as peculiar to vegetables ; being composed 

 of oxygen, hydrogen, and carbon alone ; and being identical with the material of which the greater part of their 

 issues is composed. Even the tunics of the solitary Tunicata are found to contain a large proportion of the same 

 material. The presence of this substance probably depends upon its abundance in the food of these animals 

 which appears to consist of particles of sea-weeds, and also of a number of vegetable bodies (formerly regarded 

 as animalcules) of extreme minuteness, which float in the waters of the ocean and are drawn in by the ciliary 

 current. 



Although at first sight the peculiarities of these animals might seem to detach them from the other classes of 

 the Molluscous sub-kingdom, yet the separation is not so wide as might appear. If the membranous, cartilagi. 

 nous, or leathery integuments of an Ascidian were to be converted into a hard shell, symmetrically divided into 

 two plates or valves, held togejjier by a hinge on one side, and open at the other so as to expose the mantle, 

 whilst the two orifices protruded at one extremity, it would present the closest similarity with many bivalve 

 shell-fish. The similarity would be in many respects closer, were a Salpa to be thus transformed ; since its 

 branchial leaflets bear a near resemblance to the branchial lamella? of the Conchifera. On the other hand, the 

 gemmiparous production, which is so remarkable a feature of this class, connects it with Zoophytes ; as does 

 also the peculiarity of its circulation. 



