PORPITA. 123 



the central polypite does not present that arrangement of eight radially disposed, cleft-like 

 depressions described and figured in P. Mediterranea. On the contrary, it exhibits a great 

 number (some forty) of scptiform membranous bands, which radiate between the centre and 

 the periphery of the roof of the polypite, becoming deeper as they go, until, at the circum- 

 ference, tlieir height is equal to the tliickness of the distal layer of the hydrosoma. 



Similar but very much smaller bands, not exceeding seven or eight in number, appeared 

 to me to be developed in the roof of the gonoblastidial polypites, but it was not easy to assure 

 oneself of the fact. 



The hydrosoma exhibits, as in Porpifa Mediterranea, a thin proximal, and a thick distal, 

 layer, between which the pneumatocyst is inclosed. As the other parts are all disposed 

 in relation to the structure of the last-named organ, it will be convenient to consider its 

 structure first. 



The proximal or upper surface exhibits a beautifully regular, radiating striation, each 

 stria .terminating in a little projection at the periphery of the disc, whicli thus acquires 

 somewhat the appearance of a toothed wheel. In the circumferential half, or thereabouts, 

 of the surface of the disc, these striae are the expression of ridges, with intermediate valleys, 

 which attain a height and depth nearly equal to half the total thickness of the disc ; 

 but, towards the centre, the inequalities gradually diminish, the valleys becoming filled 

 up and the ridges covered over, until, in the central half of the proximal surface of the disc, 

 the general surface is even, and would be quite flat and smooth, if it were not for the 

 numerous conical tubercles scattered over it. 



These tubercles attain a diameter of one sixtieth of an inch at their base, and one 

 fortieth of an inch in height, and they are disposed without reference to the direction 

 of the radiating ridges. The apparent radiating striation of the central region of the pneuma- 

 tocyst is due only to the shining through of the deeper structures. 



A radial section of the pneumatocyst (woodcut A, p. 126) shows that its proximal or 

 upper wall (/), which is not more than j^th of an inch thick at the periphery, gradually 

 increases in thickness towards the centre, until, in the middle, it attains a diameter of one 

 twenty- eighth of an inch, or, in other words, has twelve times the thickness it possesses at 

 the circumference. 



On the other hand, as it passes into the distal or inferior wall, the substance of the 

 pneumatocyst becomes thinner and thinner, until, over the whole distal surface, it has a 

 thickness of less than inioth of an inch. Where thin, it appears structureless ; but where 

 thick, it is obviously composed of a great number of superimposed lamellae. 



I conclude from this structure that the pneumatocyst has originally very thin walls, 

 to which very little, if any, addition is made on the distal surface ; while, as the animal grows 

 and becomes older, new layers are constantly added to the whole of the proximal wall of 

 the pneumatocyst. 



The concentric chambers of the pneumatocyst are more than fifty in number, and 

 the septa which divide them from one another are thicker where they join the proximal 

 layer of the pneumatocyst. 



The central chamber is octagonal, and has a diameter of one thirty-sixth of an inch. 

 Viewed with a low power, it seems to be divided into eight compartments by as many 

 ribs, which unite in a circular disc in its centre, and radiate thence to its angles. In the 



