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



Cystous. — Numerous Physonectse, mainly the Maerostelia (Apolernidaa, Agalrnidse, 

 and Forskalidaa), possess cystons or anal vesicles, excretory polypites which occur in no 

 other order of Siphonophorse. They have hitherto been confounded with the similar 

 palpons, although some authors have observed the distal opening, by which fluid and 

 crystalline excretions are ejected. The cystons differ from the similar but smaller 

 palpons essentially in the possession of the distal opening, which may be closed by a 

 muscular sphincter, and with respect to its function called an anus (PI. XV. figs. 8, 9, yo). 

 They seem to differ further in the peculiar structure of the wall, which is often intensely 

 coloured (PI. XVIII. fig. 2, y) and glandular, at least in the distal part. The characteristic 

 hepatic glands of the siphons, however, are wanting. The cystons are therefore excretory 

 polypites, which in morphological relations are intermediate between the digesting 

 siphons and the feeling palpons. I have never observed more than a single cyston in 

 each monogastric cormidium. In the polygastric cormidia of the Apolemidae the number 

 of cystons seems to correspond to that of the siphons. 



Privacies. — Usually in the Physonectse each single palpon, as well as cyston, is 

 provided at its base with a long palpacle, or a simple " accessory tentacle." It is a very 

 slender, never branched, cylindrical tubule, the thin wall of which contains small 

 cnidoblasts and palpoblasts. It is in perpetual motion, and its function is mainly 

 sensory. Perhaps these feeling filaments (" Tastfaden" ) are generally distributed among 

 the Physonectse; they are not observed in various genera, but it may be possible that 

 here they have been either overlooked or lost accidentally. 



Gonodendra. — Nearly all the Physonectse have monoecious corms, male and female 

 gonodendra being developed from the same trunk. There are two exceptions only where 

 the corms are dioecious : — Athoralia among the monogastric, and Apolemia among the 

 polygastric Physonects ; in these two genera each corm bears either male or female 

 gonodendra. 



The ordinate cormidia are usually monoclinic, each provided with two gonodendra, a 

 male and a female, which arise separately from the node of the trunk (PL XVIII. fig. 2 ; 

 PI. XX. figs. 9-16). Usually here the female is placed more proximally (near the palpon) 

 and the male more distally (near the siphon). These gonodendra may be called distylic, 

 since their stems or gonostyles are two independent branched palpons. There occur, 

 however, sometimes monostylic gonodendra, where the basal part of the single branched 

 gonostyle bears female, and the distal part male gonophores. This is the case in 

 Forskalia (PI. X. fig. 21 ; compare Kolliker, 4, Tab. ii. fig. 1). 



The irregular cormidia are sometimes monoclinic, at other times diclinic ; in the 

 former male and female gonodendra occur scattered over the same internode ; in the 

 latter arising separately from different internodes. In many Physonectas with irregular 

 cormidia (mainly Halistemmkko) very numerous gonodendra of both sexes are scattered 

 irregularly along the whole trunk. 



