HOMOLOGIES. 



197 



Continuing to take the Hydroida as a standard of comparison, the other liydrozoal orders 

 may be now contrasted with them. If tlie atrium (woodcut, fig. OG, b'), or tliat portion of tlic 

 somatic cavity which hes at the base of the manubrium in a hydroid medusa, be expanded 

 laterally (woodcut, fig. 09, b'), and the ectoderm of its floor be projected along four or eight 

 symmetrically disposed radiating lines into as many thick pillars (woodcuts, figs. 69 and 70 o, o) 

 which converge towards the axis and there meet the manubrial extension of the cavity, while the 

 thin intervening portions of the floor between the pillars become developed into generative 

 pouches ((/), and the velum or perforated diaphragm which stretches across the codonostome in 

 the hydroid disappears, we shall have the hydroid medusa converted, in the more essential 

 jjoints of its structure, into a discophorous medusa (woodcuts, figs. 69 and 70). 



Again, a Li/cernaria (woodcuts, figs. 71 and 72) may be conceived of by imagining a Hydra 

 with four tentacles to have these tentacles expanded laterally until their sides meet and coalesce, 

 while the hypostome {b) still remains free, the proximal portion of the body continuing to form 

 a peduncle of attachment {p), and generative sacs {d) becoming developed on each side of the 

 partitions formed by the coalescent sides of the tentacles. 



Fig. 71. 



Dli 



ramiitic loni^itudinal sectiou of Lucernarla. 



a, Ciroumoral disc ; a', marginal tentacle ; i, hypostome ; b\ 

 somatic cavity ; c, aperture by wliich the chambers of the circum- 

 oral disc communicate with one another across the distal eud 

 of the partition ; d, generative bands ; p, peduncle ; z, teutacula- 

 like processes of the inner surface of the somatic carity. 



Fig. 72. 



Diaijramatic transverse section of a 

 Lucernaria across the oircuiuoral disc 

 and hypostome. 



a, a, Chambers of the disc ; b, hypo- 

 stome ; d, generative bands. 



Lastly, the Ctenophora (woodcuts, figs. 73 and 74) admit of an obvious comparison with a 

 hydroid medusa. In order to understand this we must keep in mind the presence in the hydroid 

 medusa of an atrial segment of the somatic cavity. This is formed by that portion of the somatic 

 cavity which is immersed in the substance of the umbrella at the base of the manubrium, and 

 from which the radiating canals proceed (see woodcut, fig. 66, b'). The hydroid medusa thus 

 admits of a division by a transverse plane into two regions — an alrini region (r), which corre- 

 sponds to the solid summit of the umbrella with the parts therein contained, and a mmmhriaJ 



between the two groups. In referring to the parts of the Siphonophora I have also employed the 

 terminology proposed by Huxley for this order. See bis 'Oceanic Ilydrozoa,' page 8, &c. 



