ME. P. H. CAEPENTEE OX THE GENUS ACTINOMETRA. 81 



arms of tlie primitive oral coelom — the upper or ventral portion of which gives rise to 

 the suhtentacular canals of both arms and disk. 



The dorsal or aboral coelom of the pentacrinoid larva lies beneath the annular 

 mesentery, and forms the dorsal half of the primitive circumvisceral coeloin, long before the 

 alimentary canal is sufficiently convoluted to give rise to a distinct intervisceral coelom. 

 Like the oral coelom it sends off radial extensions into the developing arms, but beneath 

 the horizontal partition extended from the mesenteric bands, and these become the cceliac 

 canals. Its dorsal prolongation gives rise to the cavity of the centrodorsal piece, and 

 ultimately to the central canals of the calcareous segments. Both of these, together with 

 the plexiform space between the first radials and the cceliac canals converging to it, are 

 therefore, like the greater part of the circumvisceral coelom, derived from the right peri- 

 toneal diverticulum of the primitive digestive cavity ; while the left one gives rise to the 

 suhtentacular canals of the disk and arms, and to the ventral portion of the circum- 

 visceral coelom. The primitive distinction between the oral and the aboral ccelom of the 

 larva, indicated by the mesenteric fold, becomes, however, gradually obliterated by the 

 development of numerous similar septa of connective tissue, and by the growth of the 

 alimentary canal and its consequent winding. 



The axial canal, continuous above with the oral, and below with the aboral ccelom, is 

 produced by the limitation of the central space left by the coiling of the intestine around 

 the stomach ; while the remainder of the spaces between the coils become the inter- 

 visceral ccelom, which is therefore not developed to any extent until after the cceliac 

 canals of the rays have been extended from the primitive aboral coelom. 



(§ 51) All the species of Antedon do not agree with Ant. rosacea and Ant. celtica in 

 the great inclination of the distal faces of the first radials to the vertical axis of the calyx, 

 so that these faces enter into the composition of the ventral aspect of the radial pentagon. 

 In a new and undescribed Antedon from the Philippines this inclination is very slight; 

 and in a view of the pentagonal base from above but little more is seen than the proper 

 ventral faces of the component radials. In this respect, therefore, forms such as these 

 present an approximation to Actinometra (PI. V. fig. 1, PI. VI. figs. 5, 12, 23), in which 

 the distal faces of the first radials are nearly or quite vertical, and do not at all enter 

 into the composition of the ventral aspect of the pentagonal base, which consists simply 

 and entirely of the five adjacent ventral faces of the component radials. 



In Ant. rosacea (PL IV. fig. 17) these form a five-pointed star, the central surface of 

 which slopes rapidly downwards and inwards as the inner wall of the central funnel (F) ; 

 while its five rays correspond with the divisions between the component radials, and are 

 bounded by the large ridges which form the upper and outer margins of the two adjacent 

 muscular fossa? (/) of every pair of contiguous radials (PI. IV. fig. 12 a, g x ). The sutures 

 between the radials are marked by slight depressions of their ventral surfaces, and these 

 are completed by the ridges at their sides into the ventral interradial furrows already 

 described (PI. IV. figs. 1, 17, v.i.f) ; they occupy the five rays of the star, and alternate 

 with the five shallow depressions (v.r.f) lying in the centre of the ventral faces of the 

 first radials. In Actinometra, as will be seen further on, these depressions become very 

 marked ; but in Ant. rosacea they are hardly deep enough to be called furrows, and are 



