f)18 MR J. D. MACDONALD ON THE MORPHOLOGICAL RELATIONSHIPS 



The end of each division is well rounded, and closely applied to the ectoderm on 

 either side of the ganglion, but I have always found it diflBcult to detect the so- 

 called anal openings, though I have once or twice observed the escape of matters 

 from within the tube at this part. The least I can say, however, is, that they are 

 by no means so definite in nature as they are represented to be in figures and 

 descriptions. 



Professor Huxley first pointed out the striking original difference in the in- 

 testinal flexure in Brachiopoda and Polyzoa, as compared with that in Tunicata. 

 Thus, in the two former, the intestine is simply flexed forward {i.e., towards the 

 nervous ganglion or '■'■neurcdly"), while in the latter it is at first flexed backwards 

 {i.e. towards the heart, or hsemally), and subsequently turns forward to terminate 

 neurally in the " atrium.'" In Cydippe, therefore, the intestine is straight ; in the 

 Polyzoon and Brachiopod, it is once flexed (forwards), and in the Tunicary it is 

 twice flexed (backwards and forwards). 



The nervous ganglion appears to hold some relation to the conditions just 

 noticed ; thus, it is nearly midway between the branchial and cloacal openings in 

 the Tunicary, between the oral and anal orifices in the Polyzoon^ and at the 

 posterior termination of the intestine in Cydippe. 



The homologies of the ciliated bands and tentacula of Cydippe have not yet 

 been satisfactorily determined, though it does not appear improbable that the 

 former organs represent the tentacula of the Polyzoon or of the Brachiopod having 

 become retroverted, and connate as it were with the body, as next in order of 

 suppression. 



The configuration of the genus Cestiim is strongly in favour of this view, and 

 if the possibility of such be admitted, then the retractile racemose tentacula may 

 be regarded as pallial. If, on the other hand, the position here supposed in 

 relation to the ciliated bands of the Ctenopliora be doubted without valid reason, 

 I can only say that it is not at all more remarkable than the modifications of the 

 ambulacra in the EcMnodermata, passing from Asterias to Spatangus for example. 



The branchial tentacula in the Tunicata may be regarded as ecclusory and 

 perhaps prehensile, while respiration is effected by their pharyngeal apparatus, 

 and the same office is probably also exercised by its homologue the oral circle of 

 tentacula in the Polyzoon., and the so-called cirri of the Brachiopod. In the case of 

 the latter, however, it may be subserved also by the pallial sinus system, which is 

 ciliated within, and thus enabled to circulate the contained corpusculated fluid. 

 The renewal of the water passing over it is of course chiefly brought about by 

 the action of the vibratile cilia clothing the double row of tentacula (" cirri")- 

 There is apparently, to my mind, but a short step from these latter organs to the 

 ciliated bands of Cydippe, in which they are both locomotive and respiratory, 

 being in close relation with the aquiferous system ; and it must be remembered, 

 that this, like the pallial sinus system of the Brachiopod, is not only lined with 



