28 



THE HYDROMEDUSAE 



gonotheca, seated upon a peduncle continuous with the opercular 

 plate. This type of gonophore has been termed a meconidium. 



2. In Calycella lacerata a blastostyle and gonophore at first lie 

 inside a gonotheca of the usual type ; the gonophore when nearly 

 mature migrates up the blastostyle, and projecting beyond the 

 mouth of the gonotheca, secretes a spherical gelatinous cyst, the 

 acrocyst, in which it completes the maturation of the generative 

 cells. In Sertularia pumila the gonophore, having formed the 

 acrocyst as above described, withdraws again into the gonangium, 

 leaving the ova behind to develop (Fig. 36). 



FIG. 35. 



FIG. 



FIG. 37. 



35. Gonotheca of Obelia. From the central blastostyle are budded numerous gonophores, 

 each of which becomes a free-swimming medusoid. (After Allman.) 



36. Diagrammatic section of the gonophore of Sertularia pumila. A, the acrocyst, con- 

 taining ova ; B, coelenteron of the blastostyle ; G 1 , the first gonophore (sporosac), which has 

 formed, and retired from, the acrocyst, connected with the perisarc on the left by strands of 

 tissue (gubernacula) ; G 2 , the second gonophore at an early stage of formation on the blasto- 

 style ; OP, opercular plate, an expansion at the distal end of the first gonophore ; P, perisarc of 

 the gonotheca. (After Weismann.) 



37. Phylactocarp (corbula) of Aglaophenia attenuata, side view. At the point of origin of 

 the phylactocarp from the hydrocladium is a hydrotheca with the usual three nematophores ; 

 the phylactocarp itself is composed of ribs carrying numerous nematophores ; the ribs figured 

 arch over to meet those of the other side, and cover the gonophores (not shown in the figure). 

 (After Allman.) 



3. In Sertularia rosacea, in which the gonophore is also 

 borne on a blastostyle, the opercular plate sends out eight hollow 

 arms, consisting of the usual body layers ; these, projecting out- 

 wards beyond the mouth of the gonotheca, secrete eight flat spines 

 which bend inwards to serve as a marsupium for the reception and 

 protection of the acrocyst. 



4. Another method of protection for the gonophore is found 



