16 ARTHUR DENDY. 



I have seen no thread-cells with the threads everted, and 

 have not been able to make out any details with regard to 

 the thread itself. No barbs were visible in my preparations. 

 Smaller thread-cells, in various stages of development, lie in 

 the deeper parts of the ectoderm. 



4. The Medusoid. 



(a) Structure. — Although no free-swimming medusfe 

 have as yet been observed, there can be little doubt that 

 they normally separate from the parent hydroid. As already 

 pointed out, they exhibit movements of contraction while 

 still attached, and separate very readily in the process of 

 killing and preserving. Moreover none of the medusas, 

 which were found attached to the hydroid in large numbers, 

 were sexually mature, and the largest were only about 1 mm. 

 in longer diameter of the bell. 



In the largest examples the bell is considerably deeper 

 than wide, and nearly square, though with rounded angles, 

 in cross-section (figs. 22 — 24). The mouth of the bell is 

 still very narrow (fig. 23), probably expanding considerably 

 later on. It is surrounded by the velum, around which the 

 margin of the bell has grown out into four arms or lobes, 

 arranged in the form of a cross, per-radially, corresponding 

 to the angles of the bell. Each of these arms bears five 

 tentacles arranged in a very peculiar manner— a pair of 

 larger ones, a pair of smaller ones, and a very small odd one ; 

 the largest being furthest from the mouth, the odd nearest to 

 the mouth, and the remaining pair intermediate in position, 

 as shown in fig. 23. All the tentacles are short, even in 

 the living animal, and they are only very slightly if at all 

 swollen at their extremities. It is possible that the number 

 of tentacles increases as the medusa grows older, but their 

 peculiar and definite arrangement seems to indicate that the 

 full complement is already present. The tentacles are filled 

 with solid endoderm formed in the usual manner, while the 

 ^rms or lobes upon which they are borne are characterised 



