THE ACTINIANS OF PORTO RIOO. 
353 
Gonidial grooves are only feebly indicated above, but are more strongly developed below. The 
walls at the opposite extremities are slightly depressed between the pairs of directives, the filiation is 
a little stronger, and gland cells and nematoblasts are less numerous than elsewhere. The nuclei of 
the ectodermal supporting cells form a broad zone which stains very deeply; a nerve layer is also 
displayed, but no muscle fibrils. Many nematocysts and granular gland cells with yellowish contents 
occur, in addition to the supporting cells. 
In serial transverse sections the stomodfeum is seen to terminate first along its lateral walls, the two 
ends, with the directive mesenteries attached, being prolonged for some distance farther, independently 
of one another. 
Twenty-four pairs of mesenteries, including two pairs of directives, are present in the polyp 
sectionized transversely. Twelve pairs, representing the first and second orders, are united with the 
stomodseum in the upper part of their course, and twelve alternating pairs, representing the third 
order, are free throughout and extend but a short distance from the column-wall. Of the twelve 
complete pairs, six alternating pairs — the second order — become free some distance before the lower 
termination of the stomodseum is reached; the remaining six pairs — the first order — extend practically 
all the way. Thus in the upper stomodseal region two cycles of mesenteries occur, twelve pairs in 
each, while in the lower region are three cycles, with the formula 6 6 12. The directives have the 
shortest transverse course of all the complete mesenteries. 
The mesenteries are narrow and wide apart in the upper half of their course, but they become 
broader below, their free edge is greatly folded, and they more nearly fill the gastro-ccelomic cavity. 
The musculature is feebly developed above, but below the mesoglceal plaitings become stronger 
and mainly circumscribed in their arrangement (fig. 39). As the lower region of the polyp is 
approached a strong mesoglceal pennon, supporting the parieto-basilar muscle, is present on the face 
opposite the retractor muscle. The mesenterial mesogloea increases much in thickness from above 
downward, and bears small stellate cells. Everywhere the endoderm is a very narrow layer, its cells 
being more or less charged with blackish pigment granules. The serial transverse sections indicate 
the presence of the inner mesenterial stomata on all the twelve pairs of perfect mesenteries, but I have 
not been able to establish an outer or parietal series. Basilar muscles occur, represented in fig. 37. 
As already remarked, the six pairs of second-cycle mesenteries cease their connection with the 
stomodaeum in advance of the first-cycle mesenteries, and some of the members of the latter cycle also 
become free slightly in advance of the others, while the directives continue their attachment over the 
greatest distance. The separation of the mesenteries from the stomodseum, as revealed by serial trans- 
verse sections, takes place in such an order as to suggest certain morphological considerations. The 
six pairs of second-cycle mesenteries do not become free simultaneously, but the pair on each side of 
what may be regarded as the ventral directives of the polyp are free in advance of the middle lateral 
second-cycle pairs; then the middle pair on each side become free, and finally the dorso-lateral pairs. 
The relationships are represented in fig. 40, taken from one of the sections. The ventro-lateral pairs 
are altogether free; the medio-lateral pairs are just becoming free, starting at the middle of the concave 
margin, and the dorso-lateral pairs as yet show no signs of separation. Thus, in the order of separation 
of the mesenterial pair’s from the stomodreum, a regular succession from the ventral to the dorsal 
aspect of the polyp occurs, the process taking place simultaneously in the corresponding pairs on each 
side. Sirch a relationship may be taken to indicate that the mesenteries are older and further developed 
dorsally than ventrally. 
The studies which have been carried out upon the development of the second cycle of mesenteries 
of the Actiniaria indicate that the order of appearance is paralleled by the above results obtained 
from serial transverse sections of probably a mature polyp. For, in general, it may be assumed that 
where a mesentery jxjssesses diminished importance it represents a more recent development. 
It is usually found, both in Actiniarian and Madreporarian polyps, that the second-cycle mesen- 
teries are developed in unilateral pairs on each side of the polyp in a progressive manner, which may 
be either from the dorsal to the ventral aspect of the polyp, or vice versa. The fact that the dorsal 
mesenteries of the second cycle in Bunodomma retain their connection with the stomodseum longer 
than the middle pairs, and these again than the ventral pairs, would indicate a progressive development 
of these pairs from the dorsal to the ventral aspect. The successive development of the pairs of 
second-cycle mesenteries from the ventral to the dorsal aspect of the polyp I have found to be 
