136 THE VOYAGE OF H.M.S. CHALLENGER. 



One result of this flattening of the colony in the more modified LeptocUnids is that 

 there being no longer room for the Ascidiozooids to lie with their long axes at right angles 

 to the upper surface of the colony, they have become modified in one of two ways. In • 

 some cases they have simply come to lie irregularly in the colony, being inclined at 

 various angles to the surface ; Init in other cases an interesting change has taken place, 

 the antero-posterior axis of the thorax has remained vertical, the primitive position, 

 but the abdomen has become bent upon the thorax so as to point dorsally.^ Such a 

 change in the body of the Ascidiozooid would doubtless allow the thorax, and therefore 

 the l)ranchial sac, to remain of large size, while the colony was becoming gradually 

 thinner and thinner ; and in some species {e.g. LejJtocUnujn mosdeyi), the flexure has 

 gone to such an extent that the abdomen does not extend .behind the thorax, but 

 projects at right angles dorsally from its posterior end. Any further bending 

 after this condition had been reached would be useless. It may be noted that this 

 modification produces an arrangement of the Ascidiozooid which shows a superficial 

 resemblance to that seen in the Botryllidse when the abdomen seems to lie alongside 

 the thorax. The two cases are really, however, entirely difi"erent, as there has been no 

 flexure of the body in the Botryllidse, and there is no included test or double fold of 

 mantle in the angle between the thorax and the abdomen, as there is in the case of 

 the modified Leptoclinids. 



In the ancestral Diplosomidfe the reproductive organs have remained in a more 

 primitive condition than in the Didemuidse, and the vas deferens has become straight. 

 The testes are usually two in number, and are therefore in an intermediate condition 

 between the numerous spermatic vesicles of the ancestral Distomidaj and the single 

 large testis of the Didemnidse. The property of producing calcareous spicules in the 

 test has l^ecome gradually lost in the Diplosomidge. Spicules are stiU found in the 

 upper layer of the colony in Dvplosomoides,^ but have disappeared in the genus 

 DiplosomrA. As a result the test has become softer and more transparent, and the 

 system of canals and cavities in connection with the common cloacal apertures has 

 become so increased as to greatly reduce the amount of test in the colony. Dij^Jo- 

 somoides is less modified than Diplusoma, and may lie represented by a side branch 

 from the ancestral DiplosomidaJ. 



The line L. {table, p. 150), which diverged fi-om the common ancestors (K.) 

 of the Didemnidas and Diplosomidge, retained the ancestral condition of the male 

 reproductive organs found in the Distomida^ along with the partially coiled arrange- 

 ment of the vas deferens which became emphasised in the Didemnidas. At the same 

 time the colony apparently became detached, and its upper surface sank in so as to 

 produce an axial cavity, the lining of which is really morphologically a part of the 



1 See this Report, Part II., pi. xxxvii. fig. 10. 



2 This Report, Part II. p. 309. 



