50 



MORPHOLOGY. 



a jjrotective nidus for the developing ova cannot be confounded with a true acrocyst, which is 

 found only in the calyptoblastic hydroids. 



In the cases above described the acrocyst is destitute of any further covering, and has its 

 walls with their gelatinous investment freely exposed to the surrounding water. In Sertularia 

 rosacea, S. fallax and S. tamarisca, however, an additional covering is provided for the acrocyst, 

 and there is thus formed a curious and coniiilicatud receptacle, in which the ova, as in a sort of 



Fig. 22. 



Fig. 21. 



Female gonangiuin, with acrocyst 

 of Seriularia pumila. 



a, gonangivmi; i, blastostyle ; c, 

 opercular summit of blastostyle ; 

 d d, cffical offsets from the summit 

 of the blastotyle; e, gonophore 

 after having discharged its contents 

 into the acrocyst ; J", spadix ; ^, 

 proper sac of acrocyst; h, external 

 gelatinous investment of acrocyst; 

 i, ova contained in acrocyst ; Ic, 

 young ova in blastostyle. 





Female gouangiuin with acrocyst of Cali/cella lacerata. 



A, Young gonangiura, with its contents before the formation of 

 the acrocyst. B, older gonangiura, bearing an acrocyst. 



a, gonangium ; b, blastostyle ; c, opei'cular summit of blasto- 

 style ; d, membrane passing over the gonosac while still confined 

 within the gonangium ; e, gonosac still within the gonangium, and 

 containing ova which surround a lobed spadix; y, proper membrane 

 of acrocyst j its gelatinous investment. Within the acrocyst are 

 seen ova already somewhat advanced in development, and the 

 spadix depressed towards the bottom of the sac by their enlarge- 

 ment. 



iiiarsupium, pass through certain early stages of their development, previously to being discharged 

 into the surrounding water. 



The nature and morphology of this receptacle in Sertularia rosacea (woodcut, fig. 23) will be 

 best understood by tracing its development. The young female gonangium (woodcut, fig. 25) 

 is a conical body, with eight slightly projecting longitudinal ridges, and with tlie broad end of 

 the cone constituting the distal end or summit of the gonangium, A blastostyle occupies its axis, 

 having upon its sides, one over the other, the young budding gonophores, and expanding at 



