REPORT ON THE TUNICATA. 



13: 



outer surface of the colony. This produces the ancestral form from which the 

 remarkable Ccelocormus huxleyi (see Fig. 23, B.) was probably derived. 



B 



Fig. 23. — Diagr.ims showing the rehxtions between A. a typical Compound AsciJian ; B. Ccelocoi-mvs; C. Pyrosnma. 

 In all cases the colonics are represented in longitudinal section, and cl. indicates the opening of the 

 common cloacal cavity. 



Ccelocormus is a most valuable transition form, between an ordinarj- Compound 

 Ascidian {e.g., one of the Distomidje, Fig. 23, A.) and the remarkably modified Pyrosoma 

 (Fig. 23, C). It is not attached, but is prol)ably not free-swimming. It has a large axial 

 cavity, like that oi Pyrosoma, opening to the exterior at one end of the colony (see Fig. 

 23, B.) ; l)ut this cavity does not receive the atrial apertures directly as it does in the case 

 of Pyrosoma, but by means of atrial passages like those found in many Compound 

 Ascidians (compare A. and B. Fig. 23). There is a single large common cloacial aperture 

 placed on a projection at the lower end of the axial cavity, and with this all the 

 atrial apertures of the Ascidiozooids communicate by means of canals penetrating the 

 common test. 



From an ancestral form allied to Ccelocormus (Fig. 23, B.), Pyrosoma was, I 



consider, derived (see tal)le, p. 150), l)y slight changes in shape, resulting in the 



formation of an elongated hollow cylinder, and by a modification in the relations of 



the Ascidiozooids, so that they came to open independently into the large axial cavity, 



which is thus virtually converted into a huge common cloacal cavit}-. Pyrosoma is 



free-swimming, and the Ascidiozooids have acquired light-producing organs placed 



laterally on their anterior ends. Uljanin ' considers that Pyrosoma is related to the 



Compound Ascidians ; Ijut he places Distcq^lia—whidi I regard as a typical member of 



the family Distomidte, allied to Colclla — as the connecting form. I have already 



shown - that there is no essential difference in process of gemmation and iu life history 



' Fauna und Flora d. Golfes v. Neapel, Monogr. ^., Doliolum. - See before, under Pymsoma, p. 24. 



(zooL. CHALL. Exr. — r.\RT Lxxvi. — 1888.) Gggg 18 



