OCTINEON LINDAHLI. 467 



(i) The majority have very thin laminae of mesogloea; very few 

 of them ever reach the stomodseum^ and then only as minute 

 ridges on its surface which do not stretch across the coelenteron. 

 They carry neither filaments nor generative organs, and their 

 musculature is so slightly developed that it is impossible to 

 make out whether they form '^pairs^' according to the ordinary 

 standard or not. (ii) Of the twelve primary mesenteries, two 

 (5, 5) are only distinguishable from the former set by their 

 greater length, by the longer ridge which they make on the 

 stomodseum, and by their position with regard to the other 

 primary mesenteries. They are devoid of filaments, generative 

 organs, and well-developed musculature. (ill) Two others 

 (3, 3), of similar length to those last mentioned, and, like 

 them, represented on the stomodseum only by a central ridge, 

 have at the free edge of the peripheral part a considerable 

 swelling produced by plication of the mesogloea for the attach- 

 ment of muscle-fibres. They therefore carry a special pair of 

 muscles, but are devoid of filaments (and generative organs?). 

 Judged by the direction of the fibres, which run nearly verti- 

 cally in retraction, and are at right angles to the retractors, 

 these muscles appear to be " depressors," tending to flatten the 

 animal when retracted. They have much the same appearance 

 as in fig. 11 for their whole course throughout a transverse 

 series of sections, (iv) The remaining eight mesenteries are 

 of a remarkable type, the key to which appears to lie in the 

 fact that the retractor muscle-fibres are shifting off" from the 

 mesenteries, and becoming separate strong muscles with belly 

 and " tendon.^' 



The sections schematised in figs. 14 to 18 are made from 

 camera drawings of the same mesentery and muscle at diS'erent 

 heights, and are from a "transverse" series, that is, in a plane 

 parallel to the pedal disc. They should be compared with the 

 restoration in fig. 23, it being borne in mind that the latter for 

 clearness^ sake represents the animal in a slightly less con- 

 tracted condition. The mesentery consists, not of a single 

 lamina of mesogloea with pleatings for the increased adhesion- 

 surface of muscle-fibres, the usual Hexactinian condition, 



