Stndies on the Development of Larvai Nephridia. 505 



sliould not possess a ventral mesentery likc tlic truuk cavity. I 

 think De SeivYs Loxgchamps fails to recognise the true morpliolo- 

 gical imi)ort of tliis space, as it is only in a very wide sense that 

 it cau be called a sehizocoele. Schultz Claims that in the Actino- 

 trocha regenerating- after injury it always arises as two anterior 

 divcrticnla of the trunk coelom, and in some of my sections the 

 frimk coelom project» forward in tbis region in a mauner that is 

 somewhat suggestive of this, for at tbis period the cavity has not 

 appeared, wbile in stages a little later the preseptal cavity has made 

 its appearauce and rapidly increases in relative size duriug further 

 development. But the whole subject needs recousideratiou and further 

 investigation. Ikeda first described this cavity and Goodrich (11) 

 has given a very excellent and detailed description of it, but neither 

 of these authors raade any observations on its development. Ikeda 

 mentions it as "already formed in the fully developed larva of 

 every type as a space running along the inner side of the tentacular 

 circle above the septum", and sending a Prolongation into each of 

 the tentacles. Goodrich has shown that the cavity terminates dor- 

 sally in tw^o horns, which run forward in advanced larvae some 

 distance on either side of the median line towards the ganglion of 

 the preorai lobe. In any case, whatever may be the origiu of this 

 cavity, it is at least as much a coelomic cavity as that of the trunk 

 region, for the primitive origin of the coelom in Phoronis as a pouch 

 or pair of pouches from the archenteron has long been lost if ever 

 possessed by this animai. Tvs^o coelomic cavities at least can be 

 made out in the larva of Phoronis not counting the haemocoelic 

 cavity of the preorai lobe and the oollar. 



Harmer (13) has recently advanced reasons for considering the 

 preorai cavity also a modified coelomic space, he suggests that this 

 cavity never acquires full development, from the fact that this por- 

 tiou of the larva is not represeuted in the adult, being thrown off 

 during metamorphosis. In very young larvae especially those of 

 Sicilian species, pl. 31 figs. 26 and 27, I have already pointed out 

 how w^ell marked this cavity is, and in the American species 

 A. are/dtecta, Cowles (6) has found it even more pronounced. As 

 development proceeds the sac-like nature of the cavity becomes less 

 and less distiuct. And this would leud support to this Suggestion 

 of Harmeu's that it was originally coelomic. It is however at no 

 stage so well marked and distinct as the coelom sac of the trunk 

 region. Frequently in the early stages as I bave remarked (p. 495) 



