344 THE AMERICAN MONTHLY Dec, 



and other small organisms, together with oxygen in a state of 

 solution, is made to enter the pores, it then pisses through the 

 paragaster, where it parts with its food and oxygen, and takes 

 up all effete and useless matter, which it carries away by the 

 osculum. The walls of our sponge are strengthened by a skel- 

 eton of three-rayed spicules composed of carbonate of lime. 

 Had our Ascetta a row of tentacles and no pores it would have 

 all the appearance of a Hydrozooa, but unlike the Hydrozoon 

 it possesses a third layer of cells, the mesoderm, and has no 

 nematocysts. The choanaflagellate endoderm is also character- 

 istic of the Infusoria and not of the Coelenterata. 



All the sponges, except a few genera like Halisarca, have a 

 skeleton composed of calcareous or siliceous spicules, or of 

 horny fibres. Sometime the fibres enclose siliceous spicules or 

 foreign matter, such as grains of sand, or debris of shells, etc. 



The calcareous spicules are composed of carbonate of lime, 

 having all the properties of calcite, each spicule being a simple 

 crystal, although its form and general structure are purely or- 

 ganic. The siliceous spicules are composed of colloid silica, and 

 therefore can be distinguished from the calcareous spicules by 

 the application of polarized light, which has no influence upon 

 them. Sponge spicules differ greatly in form and size, and can 

 be divided into two groups, the microscleres or flesh spicules, 

 which support only a single cell, and themegascleresor skeletal 

 spicules, which support the whole sponge. Except in few cases 

 they can be distinguished from each other by their size. 



Spongin is a horny substance having tlie chemical composi- 

 tion of silk ; it generally takes the form of fibres consisting of a 

 central core of soft granular substance around which the spongin 

 is deposited in conct-ntric layers ; in some species the fibres have 

 the form of antlers ; in others they resemble the branches of a 

 tree, whilst in others they anastomose as in Euspongia officinalis, 

 and form a kind of network. In each species the fibres differ, 

 and this is made use of for classification. 



The histology of the Porifera is very interesting. The ecto- 

 derm, except in few species, consists of pavement epithelial 

 cells pinnacocytes, in the few exceptions it is locally replaced by 

 columnar epithelium. The endoderm is of the same character 

 as the ectoderm except in the Ascon, and in the fiagellated 

 chambers of all other sponges where it is formed of collared 



