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



particularly that in the processes, attains a high degree of development (PI. XXXVII. 

 fig. 8). In the pedicels of the above-mentioned species I have found that the connective 

 tissue consists of three easily distinguishable layers, — an outer which contains the large 

 calcareous plates, an intermediate of a loose and fibrous texture in which small spicules 

 of various shapes are present, and an inner layer which forms a dense, thick, hyaline and 

 elastic tunic immediately beneath which the muscular coat of longitudinal fibres is 

 situated. The tunic in question, which is distinguished by having numerous transverse 

 closely-placed wrinkles of about equal size, seems to be very intimately united with the 

 longitudinal muscular layer, because when isolating them from one another the tunic 

 bears evident impressions of the muscular fibres. I did not ascertain that the processes 

 or the tentacles of this animal possessed such an elastic tunic, which however should 

 be present ; it was probably very thin, and thus escaped my attention. In the dorsal 

 processes of Lcetmogone ivyviUe-thomsoni I have likewise observed a corresponding mem- 

 brane or tunic. 



The principal forms in which the calcareous deposits are presented in the Elasipoda 

 are, strictly speaking, only three — spicules, wheels, and plates. The body-wall is usually 

 supple and pliable because its calcareous skeleton is composed of spicules or wheels which 

 are more or less dispersed in the substance of the corium ; in Elpidia glacialis, Elpidia 

 verrucosa, Scotoplanes murrayi, Peniagone vitrea, Scotoanassa diaphana, &c, these 

 spicules lie so closely crowded and overlapping one another that the perisoma becomes 

 exceedingly brittle, whde at the same time the animal loses the power of changing the 

 shape of its body in proportion to the degree of hardness of the integument. In a very 

 few cases, Deima and Oneirophanta, the integument is strengthened by a great number 

 of perforated, larger and smaller, conspicuously overlapping plates (comp. PI. XXXI. ), 

 which constitute an almost continuous and immovable shell. 



Different kinds of calcareous bodies are often found in one and the same animal ; 

 thus Lcetmogone wyville-thomsoni is provided with wheels and simple spicules (PI. XXXI. 

 figs. 14-16), Lcetmogone violacea with wheels and. cruciform bodies (PI. XXXVI. figs. 20- 

 24), and Ilyodosmon macidatus is most particularly characteristic in having, besides 

 wheels, dichotomously branched, flat and discoidal bodies, which are partly scattered, 

 partly crowded in great numbers (PL XXXVI. figs. 12-19). 



The spicules appear variously shaped, but are, nevertheless, derived from two principal 

 forms — simple and branched (Pis. XXXII.-XXXV.). Among the former there is to be 

 noted, firstly, the C-curved type, which has hitherto been regarded as characterising 

 the genus Stichopiis, Brandt, but is now known to be present in all the representatives 

 of the genus Scotoplanes, and in some of the genus Peniagone ; secondly, the simple, 

 straight, or inconsiderably curved, spinose rods, which are found in several species, as, 

 for instance, in Scotoplanes albida, Scotoplanes globosa, &c. ; and lastly, the minute more 

 or less highly arcuated and spinose spicules, conspicuously thickened in the middle, 



