LUCERN ARI.E AND TUEIR A LLIES. 69 



stratum. Their average distance apart is, as we may judge from the drawing, 

 about equal to the measure of the thickness of the gastrophragma. 



120. The Chondrotni/ojilaj: (i', sei; also 19(5). — We have every reason to believe 

 that the fibrous prolongations wliich traverse this mass are direct emanations from 

 the muscular layer (/*'), and that the amorphous portions of the chondroniyoplax 

 constitute the interstitial cytoblastema. Tliere can be no doubt that fibres do 

 pervade this mass, and, from analogy — with facts wliich we have obs(?rved in regard 

 to the development of the chondrophys in the young Luccn-nariie, and other 

 AcalephcE — we do not h(;sitate to judge that they arc part and parcel witli the 

 muscular layer which lies on the surfoce. The choiidromyoplax (/>') and the 

 gastromyoplax (h) are here but two subdivisions of one layer. In passing we 

 would add that in the digi'tuli of young Aurelia-medusids the subdivision is much 

 less marked tlian here, in fact leaving no room to evade the conclusion which we 

 have come to in tlie present case. If this be true here, it would seem, of necessity, 

 to be so throughout the anterior parietes of tlie umbella, but the subdivision 

 is more nearly complete than in the diijituU, and there is a higher specialization 

 of the muscular layer, as evinced by the systematically arranged ridges, which 

 tends to indicate a separate function for each, the one contractile and tlie other 

 simply resilient. In the Lucernaria^ these functions incline to blend, as far as their 

 areas of action are concerned, and in the medusoid of Aurelia they are completely 

 intermingled in that respect. Altliough tlie digittdl are, like the internal inter- 

 tentacular lobules (^ 108), prolonged from the circumoral parietes of the umbella, 

 their internal arrangements are quite diverse from those of the latter; since in the 

 former the fibrilte run across the longer axis of the gelatiniform mass, and conse- 

 quently at right angles to the trend of tlieir homologucs in the circumoral chondro- 

 niyoplax. At the base of these organs the transition of the trend of their fibrillae 

 into that of the umbellar chondroniyoplax is gradual, just as it is in the genital 

 saccules, and likewise in the same manner. It should be noted, in particular, that 

 not only do they traverse the gelatiniform mass at right angles to its longitudinal 

 axis but that their trend is from one broad face of the dljitull to the other {fig. 

 100, h'). 



121. Resume. — The amount and diversity of differentiation in these bodies 

 doubtless surpasses, by far, anything of the kind in the Avliolc class of Acalephae ; 

 at least it would seem so since sonic of the highest forms which we have examined' 

 fall short in this respect. This tendency to outstrip its allies in special points, 

 while failing to equa'l them in oilier respects, is everywhere characteristic of the 

 comprehensive types. Among the Selachians we find the sharks imitating the 

 placentation of the Mammalia, in this respect, therefore, rising above Reptilia 

 and Aves. They doubtless surpass the Teliosts in every respect, except in the 

 skeleton, and they fall short there only in the lack of hardness of the bone, but 

 rise far above them in the conformation and differentiation of that system. In the 

 Annelida a complete sanguineous circulation, but without distinct walls, is the 

 premonition of a more fully developed system in the Arachnida; but this is almost 

 lost in the perisplanchnic diff'useness of the hexapod Insecta. But, if Blanchard 

 be right, the peritracheal circulation reproduces what is lost, in another, and 



