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



In contrast to the sterile muscular septa, it is the last twenty-four pairs of septa which 

 alone bear the reproductive organs (fig. 4, g), but, on the other hand, have neither muscles 

 nor mesenteric filaments ; they have, moreover, undergone retrograde formation, for they 

 merely project as small folds in the angle between the wall and the pedal disk, and only 

 extend up the wall as far as the circular constriction descril)ed above. We can distinguish 

 two parts on each septum, the free margin, which is thickened l)y the layers of reproductive 

 elements and much folded, and a thin veil-like membranous part which, like a mesentery, 

 fastens the reproductive organ to the pedal disk and wall. 



The septa of a reproductive pair are always unequal in size, and that one of them is 

 always the largest which stands next the muscular septum of the higher order. 



Enveloped in the same bit of cloth as the four specimens of Oj^hiodiscus, there was 

 a peculiar, dendritically branched body, which may possibly have belonged to one of the 

 animals as an appendage of the wall from which it had been torn away ; I shall therefore 

 give a supplementary description of it. 



The pseudo-tentacle — as I shall term it in what follows, though I do not wish to settle 

 its signification — is a very dainty, delicately-walled formation {fig. 8) ; a short basal stem 

 is almost immediately divided into numerous branches, and these, undergoing repeated 

 dichotomy, finally form a terminal bush of club-like twigs. The princijial branches fre- 

 quently anastomose, so that it is difficult to subdivide the brush of tentacles according to 

 its principal ramifications, which, moreover, form here and there small vesicular swellings. 



By the help of weak magnifying power we can make out accurately the nature of the 

 ramification and the form of the twigs (fig. 9). Each new branch is separated from the 

 preceding by a circular constriction, and begins and ends with a small swelling. One of 

 the twigs formed by dichotomy is usually behind hand in becoming branched, and this is 

 specially apparent at the ends. These present three points, as one of the twigs caused by 

 the last bifurcation only is redivided, whilst the other remains simple. 



Like the tentacles, the pseudo-tentacle contains a hoUoAV space, which is without doubt 

 an evagination of the gastrovascular system ; we can also distinguish three layers, an inner 

 layer, probably endodermal, an outer, ectodermal, and the intermediate supporting lamella. 

 AVithin the latter small fusiform cells are enclosed in a perfectly homogenous fundamental 

 substance (fig. 10). Strong, circular muscular fibres run in the ectoderm; seen from the 

 surface these caused an annulation of the branches which becomes less distinct at the ends. 

 Transverse and longitudinal sections are necessary in order to make out the position of 

 these fibres. In these sections I also observed fine fibres on the endodermal side ; they 

 were arranged longitudinally, and consequently crossed the course of the others. They 

 also seemed to be of a muscular nature. 



The epithelial layers were badly preserved, the ectodermal layer all but wanting, and 

 the endodermal merely showed a thin layer of protoplasm with scattered nuclei. 



What grounds have we for assuming that the structure described above is a com- 



