1534 THE VOYAGE OF H.M.S. CHALLENGER. 



The Concharida, the bivalved shell of which is simple, and without tubular apophyses 

 (PI. 123-125), possess a relatively small central capsule, which usually fills wp only 

 the third or fourth part of the shell-caAdty. This is the aboral or posterior part, on 

 the apex of which both valves are united by a ligament in some Concharida (PL 123, 

 figs. 8, 9). The oral or anterior part of the shell-cavity (usually two-thirds or more) is 

 filled up by the phseodium, and this is usually bifid, being divided by a frontal 

 constriction into two wings or lobes ; the dorsal wing is hidden in the upper valve of 

 the shell, the ventral wing in the lower valve ; both wings are usually united only by 

 a small central bridge, and this bridge of the phreodium is pierced in its centre by the 

 proboscis of the astropyle (PI. 124, figs. 6, 10; PI. 123, figs. 8, 9). 



The Coelodendrida have a different shape (PI. 121). Their bivalved shell is rela- 

 tively small and tiny, and bears on the two poles of the sagittal axis two conical 

 apophyses or gale«, from each of which three or four very large, dichotomously 

 branched tubes arise. The central capsule fills up the cavity of the bivalved shell 

 almost entirely, and the voluminous dark phseodium envelops both to such an extent 

 that the shell and the enclosed capsule are often hidden in it completely. Therefore 

 I arrived in my first description of Ccelodendmm (1862, loc. cit.) at the erroneous 

 conclusion that the capsule lies outside, not inside the shell. The first accurate 

 figure and description of its structure was given in 1879, by Hertwig {loc. cit., p. 99, 

 Taf X. fig. 3). The central capsule {v) is here separated from the bivalved shell 

 (m) only by a very small distance, and the oral part of both is hidden in the 

 phseodium. I find, however, in the majority of the numerous preparations of the 

 Challenger collection, the volume of the phseodium much greater, and it often envelops 

 the entire shell. 



The Ccelographida, finally, have a phseodium of the most remarkable shape, since 

 in their bivalved shell a peculiar reserve store or magazine of phseodella, which we 

 call the "phseocapsa" is developed for it (PL 126-128, g.t.m.). The bivalved shell 

 has in these most perfect PhyEODARIA a structure similar to that in the Coelodendrida ; 

 but they differ from the latter in the stronger development, and greater differentiation 

 of the two apical galeae, and the large hollow tubes arising from them. These two 

 helmet-shaped cupolse, the galese {g), which arise from the two valves on the poles of 

 the sagittal axis, are in the Ccelographida usually larger than the valves themselves, 

 and are not closed, as in the Coelodendrida, but open by a tubular apophysis at their 

 base, the nasal tube or rhinocanna (t). The apex of the galea is connected with the 

 open mouth of the rhinocanna by a single or double frenulum (6). The two nasal 

 tubes or rhinocannse (a dorsal and a ventral) lie in the sagittal plane of the body (and 

 run from the base of each galea along the anterior convexity of the valve to its oral 

 margin. Here is placed the proboscis of the astropyle, between the two opposed 

 mouths of the rhinocannse (PL 128, fig. 2). The phseodium is usually hidden entirely 



