^"isitfi^"'] PROCEEDINGS OF THE NATIONAL MUSEUM. 169 



dition of one mesentery of each pair being- niucli more lii.glily devel- 

 oped than its fellow ( 1*1. XXV, Fig. 4G). One of each ])air is quite small, 

 without reproductive organs an<l mesenterial filaments, and liardly 

 projects above the column endoderm, while its fellow is fairly broad, 

 and carries reproductive organs and a mesenterial filament. A similar 

 disparity, tliongh less marked, is to be fimnd in the pairs of the 

 fourth cycle, but I could not distinguish it in the third cycle. The re- 

 lation of the small to the large mesentery of each of the unequal pairs 

 seems to be constant, and is shown m the diagrammatic figure (PI. xxiv, 

 Fig. 4(>), It will then be seen that in the fifth cycle (v) the small mesen- 

 teries are those nearest the mesenteries of the fourth cycle (iv), while in 

 the fourth cycle the strongest mesenteries are those nearest the i)airs 

 of the first and second cycle. A few irregularly disposed mesenteries 

 of the sixth cycle could also be seen. The mesenteries of the fourth 

 and fifth cyles are gonophoric. 



As regards the nuisculature of the mesenteries, it is not very 

 strongly developed. At the base of each mesentery (PI. xxv. Fig. 50) 

 there is a strong development of muscle processes on both sides, pro- 

 ducing a basal muscle (Z>/«) similar to wliat occurs in the Edwardsiie, 

 and to a less extent in many Hexactinians. In the mesoghea of the 

 basal region of the mesenteries of the first three cycles some cavities 

 are to be obsei'ved similar to, but less highly develo])ed, than those al- 

 ready described for Bolocera occidua, and like those developed in connec- 

 tion with the parieto-basihir muscle (phm), which forms a slight pro- 

 jection on one side of the base of the mesenteries. The longitudinal 

 nuiscles cover all the muscular portion of the mesenteries in an almost 

 uniform layer, only toward the inner edge of the muscular region be- 

 coming longer and forming a rather weak nniscle pennon (PI. xxv, Fig. 

 49). The nniscle i»rocesses, es])ecially in the pennon, show a tend«^ncy 

 to be arranged in groups on more or less distinct ])luiit jii'ocesses of 

 mesoglft^a. 



Amongst the Challenger material JJi/sucUs crassmn'nis presents cer- 

 tain features of marked similarity to Acfinostolacallosa. The general 

 arrangement of the muscle cavities of the si)hincter muscle seems to be 

 identical in the two forms, and the peculiar arrangement of the 

 mesenteries of the younger cycles shows interesting similarities. 

 There are, however, certain difteren(;es in the arrangement, which 

 have made me hesitate to identify the two forms, though I am inclined 

 to believe that Dj'/.sYrc^/.s' erassicornis is to be proi)ei'ly refern^d to the 

 genus Actinostola, and that it is even probable that it may be identical 

 with A. callosa. There can be little question that its reference to 

 Milne-Edwards' genus Dijsactis is incorrect, since we know that two 

 at least of the forms referred by its author to it, />. dunnlaia (Lesueur) 

 and D. huer'mUs {— Alptasla coachii Uosse), are 8agartids, while 

 JJ. chilensis is also referred to that family by Verrill and Aiulres. If, 

 therefore, tlie forms referred to Milne-Edwards' genus are tSagartids it 



