52 



MORPHOLOGY. 



Fig. 



present altogether an appearance of great elegance (woodcut, fig. 24, e). These eight radiating 

 arms are composed of ectoderm and endoderm, and have their axis occupied by a tubular cavity, 

 whicli communicates witii that of the blastostyle. As the arms continue to elongate, we find 

 tliem next witli their free extremities bending towards one another, until finally (woodcut, fig. 

 2.3), they completely enclose a space, which becomes entirely shut in by the lateral coalescence of 

 tiie wide chitinous tubes with which the radiating processes are each invested. 



In the mean time the eight longitudinal ribs of the 

 gonangium continue themselves upon the radiating arms, 

 and ultimately extend beyond their extremities as free 

 pointed processes (y, h). Two of them, however, situated 

 opposite to one another, greatly surpass the others in size, 

 and mainly contribute to the peculiar and characteristic 

 form of the gonangiimi. Into the marsupial chamber thus 

 formed the ova make their way, enclosed apparently in a 

 proper acrocyst. 



A very similar condition is presented by Scrtularia 

 fallax. Here, however, the summit of the blastostyle is 

 prolonged into four instead of eight lobes ; but the subse- 

 quent history of these is in all essential points the same as 

 in Serttilaria rosacea. 



If we compare the structures now described with an 

 ordinary hyrandth, we shall have no difficulty in recog- 

 nising an exact parallelism ; for the tubular processes 

 which are developed from the summit of the blastostyle 

 may be regarded as homologous with the tentacles of a 

 hydrantli. They have, however, undergone a special modi- 

 fication, by which they become subservient to an entirely 

 diff'erent function from that of the tentacles of the hydranth ; 

 for, no mouth being developed on the blastostyle, they are 

 no longer prehensile organs administering to the alimenta- 

 tion of the colony, Imt, like the blastostyle itself, have 

 assigned to them functions appertaining to reproduction rather than nutrition, and are destined 

 to circumscribe a cavity for the retention and development of the ova. 



The ova would seem to continue in the marsupial cavity until they have acquired the 

 condition of ciliated embryos. 



The modification of marsupial receptacle which occurs in Serttilaria tamarisca is also very 

 interesting. The female gonangia (woodcut, fig. 26 b) are here of an oval form for about the 

 proximal half of their length, and then become trihedral, with the sides diverging upwards, 

 while the whole is terminated by a three-sided pyramid. The sides of the pyramid are cut 

 into two or three short teeth along their edges, and each of their basal angles is prolonged into a 

 short spine. 



The trihedral portion, with its pyramidal simnuit, is formed of three leaflets (y,^',y") which 

 merely touch one another by their edges, without adhering, so that they may be easily pushed 

 aside from one another by the dissecting needle, or by the embryo during its escape. 



Very yonng female gonangium of 



Sertularia rosacea, 

 tf. Cavity of the gonangium ; c, blasto- 

 style; d, opercular summit of blastostyle; 

 e, radiating lobes from summit of blasto- 

 style, about to be prolonged into arms ; 

 k, young gonopbore. 



