REPORT ON THK PENNATULIDA. 267 



of the polypes is easier to study in Virgularia than in the other genera, 

 because by making a series of transverse sections through the lower 

 end of the rachis at different levels all the successive stages of develop- 

 ment can readily be obtained from a single specimen. 



At the very bottom of the rachis there is no trace of polypes at all, 

 and at this part the fleshy substance of the rachis, which is here of 

 considerable thickness, is hollowed out to form the large lateral 

 chambers already described. 



A little higher up we get the first rudiments of the polypes. These 

 appear as transverse rows of small pit-like depressions of the superficial 

 layer of ectoderm which clothes the whole rachis (Fig. 6 dr). Each pit 

 opens by its mouth on to the surface ; its inner end, which is closed, 

 projects somewhat into the lateral chambers of the rachis, as shown in 

 the figure. Each of these pits will become the stomach of a polype, 

 the inouth of the pit remaining as the mouth of the polype. 



We have already said that the pits are arranged in transverse rows; 

 each row is situated on one of the slightly marked transverse ridges 

 which mark the commencing leaves at the bottom of the rachis ; and 

 in each row there are seven or eight polypes according to the number 

 pi-esent in the fully developed leaves of the same individual. In each 

 row, also, the polypes gradually increase in size from the dorsal to the 

 ventral surface. 



A little higher up in the rachis, i.e., at a slightly later stage of 

 development, we find the pits somewhat deeper; we find, also (Fig. 6), 

 that the lateral chambers have become divided by radial partitions 

 into smaller chambers, one for each pit, which become the body- 

 cavities of the polypes. These body-cavities grow up round the pits, 

 leaving them attached to what are now the body-walls of the polypes 

 by the eight septa or mesenteries. Round the mouths of the pits a 

 series of small buds begins to appear, the rudiments of the tentacles. 



The constrictions separating the leaves from one another become 

 more and more marked, so that the leaves gradually acquire indepen- 

 dence of one another ; the tentacles grow rapidly in size, and develop 

 along their inner borders the pinnules ; the walls of the pits, or the 

 stomachs of the polypes, become thrown into the folds characteristic 

 of the adult polypes, and the bottoms of the pits become perforated, 

 thus placing the stomach-cavities] in communication with the body- 

 cavities ; and then the extension of the mesenteries to the bottom of 

 the polype-cavities, and the thickening of their free edges to form the 

 mesenterial filaments, are all that is necessary to complete the develop- 

 ment of the polypes. 



"We shall only notice one other point : the great retractor muscles 

 of the polypes appear at a very early stage, when the stomach cavities 

 are mere pits and no traces of tlie tentacles have yet appeai-ed. They 

 are shown at about this pei-iod in Fig. 6, p. By studying the early 

 stages carefully it can be seen that these muscles are portions of the 

 great subcutaneous system of nuiscles which originally extended uU 



