584 PROF. a. J. ALLMAN ON THE 



had entered into its composition ; and we accordingly find the 

 contractility of the endocyst retained by the cord*. 



It is obvious that with these conditions there can be no inva- 

 gination or evagination of the cystid walls; and the endocyst 

 being anteriorly closely adherent to the walls of the polypide, 

 while it is quite free from the ectocyst, the polypide in the act of 

 protrusion carries out with it the whole of the anterior part of the 

 endocyst without any evagination, and in retraction withdraws 

 it without invagination into the tube of the chitinous ectocyst. 



The obliteration of the endosarcal cavity need not surprise us ; 

 for in Pedicellina a nearly similar condition exists. Here also, as in 

 JPedicelUna, the absence of special retractor and parieto-vaginal 

 muscles is a necessary result of the obliteration of this cavity. 



The contractile cord-like portion of the endocyst offers a me- 

 chanism quite efficient for the retraction of the polypide. Its 

 protrusion, in the absence of an endocystal cavity filled with a 

 perigastric fluid, is not so easily explained. It is possible, how- 

 ever, that this may be aided by the elasticity of the cord, or, as 

 Sars suggests, by the action of the great foot-like shield. 



The determination of the true import of the shield is a matter 

 of considerable difficulty ; but I believe that what we know of it 

 in connexion with the development of the polypide will tend to 

 throw light on this question. In the earliest known stage of the 

 bud, the shield already exists as a relatively very large organ. 

 Indeed in this stage we can find nothing but the short thick 

 cord-like endocyst carrying on its free end the great shield, which 

 in jB. miralilis is, according to Sars, in the form of a moderately 

 curved disk, while in R. Normani it has the curvature carried 

 to such an extent as to make it resemble the two-valved fleshy 

 mantle of a Lamellibranchiate. 



Within the hollow of the curve the polypide is gradually de- 

 veloped. The shield becomes still larger, and continues for some 

 time to exceed the growing polypide in size ; but it is at last 

 surpassed by this, and is finally reduced to the condition of a 

 mere appendage of the polypide. 



Prom this account of the origin of the shield and its connexion 

 with the polypide, it is obvious that it cannot be the homologue 

 of the epistome of the proper phylactolsematous Polyzoa. Though 

 our knowledge of the development of the epistome is by no means 



* If such be the true interpretation of the contractile cord of Ehabdopleura, 

 this cord cannot be homologous with the funiculus of Alci/omlla &c. 



