LARVA OF MEMBRANIPORA AND FLUSTRELLA. 23 



into a very expansible oesophagus, which ascends towards the aboral 

 surface; here it bends sharply downwards and joins the dilated 

 stomach which passes into the hind-gut. The anal aperture (an) 

 opens into the atrium. In the space between the mouth and the 

 anal aperture there is a paired glandular thickening (s). Near this 

 kidney-shaped organ lies a similar smaller organ (s'). OSTROUMOFF 

 has shown that when the larva begins to change into the sedentary 

 form, fixation takes place by means of this paired disc and the 

 cementing substance secreted by it, so that we may regard it as 

 the homologue of the sucker of other Ectoproctous larvae. 



The smaller organ seen near the sucker was assumed by SCHNEIDER and 

 H. PROUHO to be the transverse section of the adductor muscle of the shell. 



It is evident from the above description that Cyphonautes agrees in all 

 important points with- the typical Alcyonidium larva, from which it is distin- 

 guished chiefly by the slighter development of the retractile disc and by the 

 absence of a circular mantle-cavity surrounding that organ. In place of this 

 cavity we find an extensive surface (mantle-surface) on the aboral half of the 

 body, which secretes the two cuticular shell-valves. The oral surface does 

 not here, as in Alcyonidium, bulge out downwards, but is withdrawn into 

 the interior of the body, forming an atrium such as is also found in the 

 Entoproctous genus Pedicellina. 



Intermediate between the Cyphonautes and the Alcyonidium type we may 

 place the larva of Flustrella hispida, known through the researches of 

 METSCHNIKOFF (No. 20), BAHROIS (No. 6), and PROUHO (No. 28). In this 

 larva, the rudiment of the alimentary canal appears in the embryonic stages, 

 but degenerates later. In other respects, the development of the Flustrella 

 larva agrees closely with that of the Alcyonidium larva. . An embryo develops 

 in which the central part of the aboral surface becomes marked off from the 

 corona by a circular mantle-cavity. Only secondarily does the corona bend 

 round downwards in the form of two lateral lobes, apparently enclosing the 

 oral surface in an atrium, while the circular mantle-cavity disappears. Paired 

 shell-valves then appear and cover the whole aboral surface with the exception 

 of the very small retractile disc. A similar though less complete trans- 

 formation in the same direction is found in the larva of Eucratea chelata 

 (BARROIS). 



The researches of PROUHO have led to interesting revelations as to the 

 internal structure of the Flustrella larva. With regard to the presence of a 

 nervous system, PROUHO'S researches agree essentially with those of HARMER. 

 In the Flustrella larva, the retractile disc is connected, as in Cyphonautes, with 

 the pyriform organ by a strand of muscle-fibres. Along this strand runs a 

 bundle of nerve-fibres ; these start from the retractile disc and can be followed 

 as far as to the cells of the ectodermal furrow and of the corona. In their 

 course, single fibres of this bundle become connected with ganglion -cells. 

 PROUHO thinks that he can trace back the elements regarded by HARMER as 

 ganglion-cells to simple mesoderm-cells, and considers certain elements which 

 are provided with large nuclei as the true ganglion-cells of this region. 



When the complex organ connected with the ectodermal furrow is more 

 carefully examined, an inner cell-complex (glandular organ, pyriform organ 



