No. i.] MORPHOLOGY OF THE ACTWOZOA. 1 37 



a ventral surface, is present. It is moderately deep, and is not 

 provided at its outer extremity with lobes forming a conchula, 

 in this respect differing from PeacJiia Jiastata. 



The mesenteries are twenty in number (PI. IX, Fig. 3), and 

 are situated at equal distances from each other, but are, never- 

 theless, arranged in pairs, as may be seen from the longitudinal 

 muscles. There are two pairs of directives, one being in con- 

 nection with the siphonoglyphe, and with the exception of these 

 pairs, all the pairs have their longitudinal muscles on contiguous 

 faces, as in the Hexactiniae. All the mesenteries are perfect, 

 and consequently a section through the upper part of the col- 

 umn does not show the relation in which the various pairs stand 

 to each other. Lower down, however, this becomes evident. 

 On either side of the ventral directives come a pair of narrow 

 mesenteries (IX) ; then dorsal to these a pair much wider, and 

 with the longitudinal muscles much stronger (I and V) ; next to 

 these, a second pair of narrow mesenteries (VIII) ; then a second 

 strong pair (VI and II) ; and finally the dorsal directives. The 

 numbers with which the mesenteries are denoted indicate, so 

 far as the directives and the strong mesenteries are concerned, 

 their probable order of development, VIII and IX belonging 

 to the first cycle of paired mesenteries. The arrangement of 

 the mesenteries consequently coincides with what has been 

 described in PeacJiia Jiastata. 



The longitudinal muscles are well developed, and have the 

 form shown in Figs. 3 and 4. Upon the ventral directives they 

 only reach their full strength at a much lower level than on the 

 other mesenteries. Well-developed mesenterial stomata occur 

 in the upper parts of the mesenteries, and mesenterial filaments 

 occur on all the mesenteries. I cannot make any statements 

 regarding the distribution of the reproductive organs on the 

 mesenteries, since in the single specimen I had for examination, 

 they were in a very immature condition and difficult to make 

 out. It seems probable, however, that they are present on all 

 the mesenteries. 



H. producta can hardly be assigned to the genus Halcampa, 

 on account of the arrangement of the mesenteries ; nor does it 

 agree exactly with PeacJiia, as limited by Haddon ('89). Whether 

 the conchula is to be regarded as an important classificatory 

 feature or not seems uncertain. For the present I prefer to 



