REPORT ON THE TUNICATA. 135 



and the various species of Colella (see Fig. 22). Those are all typical Distomidse, in 

 wliich the alimentary and reproductive viscera form a mass, the abdomen, placed 

 hchind the thorax or branchial region of the body (Fig. 22, B.) As no post-abdomen 

 is present, the antero-posterior elongation has evidently not gone so far as in the 

 case of the Polvclinidfe, in feet very little change has taken place in the arrangement 

 (jf the viscera since the two groups separated at G. In many of the Distomidaj 

 {e.g. in the genera Distaplia ^ and Colella) the basal j^ortiou of the colony, consisting 

 of test only, becomes greatly elongated to form a large peduncle, upon the summit 

 of which the upper part of the colony containing the Ascidiozooids is Ijornc (see 

 Fig. 22, A.). 



Farther on the main axis of the Distomidse, after giving off at the point I. (table, 

 p. 120, or p. 150), a branch leading to the Didemnidfe, the Diplosomidag, Ccelocormus, 

 and Pyrosoma, terminates in the genus Distoma, with its closely related form 

 Cystodytes, as a short side l^ranch. Cystodytes ^ is distinguished by a modification of 

 the test, in which large disk-shaped calcareous sjMcules are produced in such a manner 

 as to form investing capsules around the bodies of the Ascidiozooids. 



The line leading onwards from I. (see table, p. 150) was occupied 1)y a series 

 of ancestral forms, in which, while the general characters of the Distomidaj were 

 preserved, some important changes were effected in the test and in the reproductive 

 organs. The test cells gradually acquired the property of producing spherical or stellate 

 calcareous spicules ; while the vas deferens assumed the spirally coiled form which is 

 so characteristic of the Didemnidae. This ancestral line gave rise to two branches, 

 one (K. in tal)le) leading with comparatively little change to the Didemnidaj and the 

 Diplosomidaa as tliey are now known, and the other (L. in talile) producing the curiously 

 modified Coelocormus, and eventually Pyrosoma. 



The ancestral Didemnidre forming the line K. must have divided into two series, 

 those leading to the Didemnidt^ proper and those leading to the Diplosomidse. On 

 the former, near the point of division, may be placed the side l)ranch leading to 

 Euccelhim, where the numljer of rows of stigmata in the branchial sac is greater 

 than three or four, thus resembling most of the ancestral Distomidai from which the 

 Didemnida3 were derived. In the family Didemnidaj the power of producing calcareous 

 spicules in the test has reached its greatest development, and the male reproductive 

 organs have become concentrated to form a single large ovate testis around which the 

 vas deferens is coiled spirally. 



The genus Didemnum is less modified than Leptoclinum, and may be represented by a 

 short side l)ranch from near the ancestral forms of the family. In Leptoclinuin the colony 

 has become greatly flattened from above downwards so as to form in most cases a mere 

 incrusting film in which the test is usually densely crowded with calcareous spicules. 



J Delia Valle, Archiv. ital. <l. Biol, torn. i. p. 19;5, 1881. - Von Draschc, Die Synascidien, p. 18, Wien, 1883. 



