REPORT ON THE CALCAREA. 19 



ostium meet by threes, by fours, and even more around the same opening. This is the 

 case, for instance, in Amphoriscus elongatus, n. sp., and, if we compare the corresponding 

 picture (PL IV. fig. 5) with that showing the inner organisation of my Leucilla connexiva 

 (PI. VI. fig. la), the way in which the Leucones have been developed from the Sycones 

 will be quite clear. What is to be regarded as an exceptional arrangement in a Sycon 

 becomes the rule in a Leucon. The invagination of the gastric surface, shallow in the 

 Sycon, becomes deep in the Leucon ; the irregularities in the disposition of some 

 of the subgastric and subdermal spicules also arise gradually, and it is not every one of 

 these spicules which retains its place between two neighbouring radial tubes ; this, more- 

 over becomes physiologically unnecessary, for a diminution in the size of the radial 

 tubes follows their rising towards the dermal surface. The invaginated part of the 

 gastric cavity, presenting in its simplest form a cone without secondary lateral sacs, 

 may produce branches in its own turn. We find this condition depicted in PI. VI. 

 fig. 2a. 



The radial tubes — here more correctly called flagellated chambers — although still of 

 cylindrical form are short and comparatively small, and though the subdermal quadri- 

 radiate and the subgastric triradiate spicules preserve their former disposition, there are 

 to be found in the parenchyma spicules scattered without any evident order. Now the. 

 ramifying of the invaginated parts of the gastric surface may go further and further • 

 hand in hand with this the flagellated chambers become smaller ; their form also under- 

 goes a modification ; elongated in the preceding cases, they now grow more or less round. 

 The intercanals, whose function is to provide the flagellated chambers with water, following 

 the modification in the disposition of these latter, already somewhat irregular in the 

 Sycones with non-articulated tubar skeleton, ramify more and more ; it becomes quite 

 impossible to find any order in the disposition of the spicules in the parenchyma ; and if 

 we examine PL VI. fig. 3a, which gives a true idea of the typical organisation of the 

 Leucones, we shall find in the species to which the illustration refers {Leuconia multi- 

 formis, n. sp.) but one trace of its origin from the Sycones, viz., the subgastric triradiate 

 spicules, which send their basal ray towards the cortex, their curved lateral rays lying in 

 the plane of the gastric surface. These triradiate spicules being in most cases very thin 

 in comparison with the stout triradiate ones of the parenchyma, no physiological signifi- 

 cation can be assigned to them, and I think there can be no other explanation of their 

 presence except that of a phylogenetic character. In Leuconia typica, n. sp., presenting 

 a form closely allied to Leuconia multiformis, these interesting subgastric triradiate 

 spicules are no longer to be found, and the spicules of the cortex, as well as those of the 

 gastric surface, are the only constituent parts of the skeleton which follow a fixed law in 

 their disposition. I do not speak of the quadriradiate spicules accompanying in certain 

 species the exhalent canals in all their windings, for they are exact homologues of the 

 quadriradiate spicules of the gastric surface. In some cases, for instance in Pericharax 



