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



to a short, club-shaped hollow tentacle, whose " sense canal " ends in an otolite sac or 

 " crystal sac " (PL XXX. figs. 4-7). The latter consists of a considerable, spheroidal or 

 oval accumulation of crystalline concrements, which have been formed in the endoderm 

 cells of the tentacle canal ; it is enclosed in a fulcral sheath covered externally by the 

 ectodermal epithelium, beset with long, stiff, auditory hairs. A peculiar tactile plate (?) 

 whose rod-shaped tactile cells bear long flagella, is usually found at the proximal base of 

 the auditory club on the axial ventral side, whilst on the abaxial dorsal side there is a 

 visible pigment pad which is considered as an eye, and sometimes encloses a lens and 

 sometimes not. These eyes appear to attain their highest development in the Pero- 

 medusse and Cubomedusse, in them we often find several eyes in each rhopalium, in which 

 a crystalline lens and a retina with a large optic ganglion may be developed (System, 

 pp. 401, 427 ; taf. xxiii., xxv., &c.,). 



§ 87. In all Medusse the muscular system is composed of two different principal 

 sections, a circular, and a longitudinal system of fibres. Both form a thorough 

 contrast, not only by their local distribution and by the direction of the course of their 

 fibres, but also by their histological nature ; the circular or transverse fibres are usually 

 clearly striated, whilst the radial or longitudinal fibres are fiat for the most part. By far 

 the largest and most important part of the two systems belong to the subumbrella which 

 functions chiefly as swimming organ. The muscular system of the umbrella margin 

 and the tentacles generally proceeds from the subumbrella. On the other hand, the 

 muscular system of the exumbrella, which is only very partially developed, is by no 

 means important. Both the transverse and the longitudinal fibres are exclusively pro- 

 ducts of the ectodermal epithelium, with which they are still most closely connected (comp. 

 above, §§ 51-53). Moreover, in some (perhaps all?) of the Medusae, weak (usually very 

 unimportant) muscles which originate from the endodermal epithelium of the gastro- 

 vascular system appear on certain parts of the body. Certain circular muscles of the 

 oesophagus and the muscles of the gastral filaments belong to these endodermal muscles, 

 which as yet have been but little recognised and investigated. Although the two sec- 

 tions of the class of Medusa? have originated independently of one another, the differen- 

 tiation of the muscular system show analogous conditions in both cases. In both the 

 circular and the radial system of fibres we can generally distinguish three sections, of 

 which the first occupies the central and proximal part, the second the middle part (the 

 true umbrella in the more restricted sense), and the third the marginal or distal part 

 (along with the marginal appendages). 



