306 HYDROID^. Part IV. 



main stem (PI. XXIX. Fig. 10, g), the pedicel [Fig. 11, c^) of the hydra, and the 

 diaphragm (c) of the calycle, are identical with the same parts in C. poterium 

 (PI. XXVIII. Fig. 3, c c" <? c*); but the contour of the calycle (PI. XXIX. Fig. 11, 

 c^), the thickness of its wall, and its border (c^), with from twelve to fourteen 

 teeth, agree with C. bicophora. On the whole, this species is a little smaller than 

 the two others, the characteristics of which it combines. It is about as frequent 

 as C. bicophora, but is likely to be overlooked, on account of its strong resem- 

 blance to the latter. 



SECTION III, 



CLYTIA (PLATYPYXIS) CYLINDUICA. 



Proles medusoidea. — The newly-born medusa (PI. XXVII. Figs. 8 and 9) of this 

 hydra has the form of a hollow sphere, from Avhich a segment, equal to one third 

 of its diameter, has been sliced off. From the centre of the bell hangs a simple 

 tubuliform proboscis [d), and from the base of this, four slender, radiating, equi- 

 distant, chymiferous tubes [e) descend along the inner face of the dome to its 

 edge, where they join a circular tube {b) which is continuous throughout the 

 circumference of the disk. The four canals have a uniform breadth from apex 

 to base, and the circular tube has a similar uniformity, but is a little broader. 

 About half way between the apex and base of each radiating canal, there projects 

 from the face of the dome a slight, oblong swelling (/), which is about twice as 

 long as, and a little broader than, the diameter of the tube. These swellings 

 represent the incipient, reproductive organs. From each of the four points of 

 junction of the radiating and circular canals, hangs a single tentacle (c), which 

 has a triangular hollow base (c^), narrowing rapidly into a cylindrical, solid, slender 

 organ of prehension. As these organs are habitually coiled up spirally, it is 

 not easy to determine their length accurately, but they seem to be capable of 

 extending several thnes the length of the bell. Midway between every two 

 tentacles, the edge of the disk bears a slight granulated swelling (e^), which is 

 open interiorly and in direct communication with the circular canal [b), precisely 

 in the same Avay as do the incij^ient tentacles of Tiaropsis (PI. XXXI. Fig. 10), 

 and on this account we infer that it is a similar organ ; in fact, from Avhat we 

 have observed in regard to the eyes, we have every reason to believe that this 

 medusa is closely allied to Tiaropsis. Just below and within the edge of the 

 disk, and half way between every tentacle and the next tentacle-bud, on each side 



