1226 



TUNICATA. 



process of formation ; the former constitutes 

 the principal mass of the excretory tuhe. The 

 internal part of the tail, which is apparently 

 a direct continuation of the substance of 

 the embryo, possesses an interior cavity, 

 and its walls are composed of two layers 

 of cells. The internal layer is formed of cells 

 of 0'012 /// diameter, rectangular, with the 

 angles slightly rounded, distinctly nucleated, 

 and containing fine yellowish granules. They 

 are very regularly arranged side by side in 

 transverse series, so that the cavity of the tail 

 is always surrounded by 10 12 cells. The 

 external layer is composed of a continuous, 

 simple layer, formed of minute cells, measur- 

 ing O003'" 0-004/", without any distinct ar- 

 rangement. It is to be observed that this tail, 

 formed simply of cells and a homogeneous 

 envelope, exhibits very active movements, 

 affording a new proof of the contractility of 

 parts composed merely of simple cells. 



The body of the embryos of Aplidium and 

 Amaroucium is formed (at an early stage) of 

 a thick, homogeneous, external layer, and a 

 yellowish mass enclosed within it. In the 

 spherical portion of the body, this mass is ap- 

 parently wholly composed of round cells, of 

 different sizes, and containing nuclei, and, 

 towards the interior, probably unaltered glo- 

 bules produced by the division of the vitellus. 

 These two elements do not compose any dis- 

 tinct organ ; but only form two layers, one 

 internal and opaque, and the other external 

 and diaphanous. In Amaroucium Nordmanni 

 there is no canal within the tail, but its centre 

 is occupied by a simple series of large rectan- 

 gular nucleated cells, producing a transversely 

 radiated appearance, visible even when mode- 

 rately magnified ; the tail has also an external 

 simple layer of minute cells, as in Botryllus. 



Savigny, also, was led by his researches to 

 regard both the ovum of Botryllus and that of 

 Pyrosoma as giving birth to several individuals 

 having already a certain order of arrangement. 

 This view, in the case of the Botryllus, is, as 

 we have seen, supported by Lowig, Kdlliker, 

 and Sars, and is considered by Van Beneden 

 as founded on fact and supported by analogy. 

 Milne-Edwards, however, is not disposed to 

 admit this conclusion ; for, in his opinion, the 

 existence of the four embryos united in a 

 circle in the Pyrosoma, and the development 

 of a single star of germs in the larval Botryl- 

 lus, do not sufficiently account for the associa- 

 tion of many such groups in the adult age ; 

 there being, for instance, in the adult Pyrosoma, 

 many hundred individuals of different de- 

 grees of development. Van Beneden thinks it 

 probable that the presumed aggregate larvae 

 produce colonies similar to themselves by fis- 

 siparous reproduction; but Milne-Edwards 

 gives it as his confirmed opinion that, from the 

 single product of the ovum, the other asso- 

 ciated individuals arise by gemmiparous repro- 

 duction only. Of this mode of generation in 

 the Botryllid(B we will now proceed to give a 

 slight sketch, again acknowledging the labours 

 of the learned Professor of the Garden of 

 Plants as the chief source of our information. 



In Diazona, Didemmtm, Botryllus, and Bo- 

 trylloides, the common test is traversed by 

 numerous ramifying prolongations of the inner 

 tunic of the individual animals, terminating 

 either simply in culs-de-sac, or swelling out 

 into germs (fig. 785.). Savigny figured these 



Fig. 785. 



Vertical Section halfway through a Mass of Botryllus, 



magnified about 6 diameters. (Original), 

 a, animals on one of the exterior surfaces of the 

 mass ; b, proliferous stolons, traversing the mass, and 

 bearing reproductive germs. 



tubular bodies, and vaguely described them 

 under the name of " marginal tubes " and 

 "vascular branches." And Delle Chiaje*, in 

 treating of Potyclinium, figures and describes 

 " vessels " which are probably these ramifying 

 canals. These membranous tubes, with their 

 terminal vesicular enlargements, are readily 

 seen during life in those species that possess 

 a semitransparent test. Milne-Edwards ob- 

 serves that in Botryllus and Botrylloides, (figs. 

 785 and 783, t, t')> each of these interior ap- 

 pendages appears at first as a little tubercle 

 on the surface of the abdominal portion of the 

 inner tunic of the adult animal. The tubercle 

 then becomes elongated, forming a tube, the 

 free extremity of which is closed, its cavity 

 communicating with the abdominal cavity of 

 the animal from which it springs. The blood 

 from the abdominal cavity circulates through- 

 out this caecal tube, with a very active double 

 current. As the tubes lengthen, they genera!!}* 

 become ramified, and soon present swollen 

 or claviform extremities. The circulation con- 

 tinues active, and before long there is visible 

 towards the summit of each terminal swelling 

 a minute granular mass, the colour of which 

 approaches that of the thorax of the adult 

 animals situated close by. A little later this 

 organised mass begins to present the form of 

 a little Ascidian, and soon afterwards becomes 

 a young animal, similar to those already occu- 



* Memoirs, second edition, torn. i. p. 34. tabl. 83. 

 figs. 13. and 15. 



