276 Mr. W. M. Holmes on 



than the skeleton spicule, and are never bound together in 

 any way. 



Fresh-water sponges, when living, are generally of a green 

 colour, more especially when growing in situations fully exposed 

 to the light, and from this cause might be mistaken for vegetable 

 growths. They are principally met with in running water (the 

 finest specimens will always be found where there is a strong 

 current), and are attached to stones, pieces of floating wood, 

 roots, and submerged branches of trees, &c. Muddy water is 

 fatal to them. 



The classification generally adopted is one based on the 

 system of Mr. H. J. Carter, F.R.S., a short account of which I 

 will now give : — 



Group Spongillina. 



Char. Bearing seed-like reproductive organs called statoblasts 

 or gemmules. 



This is divided into seven genera, viz. : — 



1. Spongilla. Gemmules surrounded by acerate or cylindrical 

 spicules alone. 



2. Meyenia (named after F. J. F. Meyen, who first discovered 

 the birotulate spicules in 1839). Gemmules surrounded by 

 birotulate spicules of a single class or type ; diameter of rotules 

 equal or nearly so. 



3. Heteromeyenia. Birotulates of two classes or types, both 

 resting by one rotule on the chitinous coat ; the less numerous 

 class longer than the other. 



4. Tubella (a little trumpet). Birotulates with rotules un- 

 equal ; the larger rotule on chitinous coat. 



5. Parmula (a little buckler). Spicules with distal rotule 

 entirely eliminated. 



6. Carteriiis (after H. J. Carter, F.R.S.) Foraminal tubules 

 prolonged, their tubules broadly funnel-shaped, or divided into 

 cirrhous appendages of varying numbers and lengths. 



7. Uruguay a (from Eiver Uruguay). Provisionally constituted 

 on the supposition that gemmules were not developed. These 

 have since been discovered by Dr. Hinde with spicules similar to 

 Meyenia leidyi, but he considers that the cylindrical form of the 

 skeleton spicules and their peculiar arrangement in the skeleton, 

 and the greater solidity of these sponges, give this genus 

 characters sufficiently distinct to mark it off from other fresh- 

 water sponges. 



Although this system of classification, based on the form of 

 the gemmule spicules, has many advantages, it may be questioned 

 whether it is, strictly speaking, a natural one, for we find even 

 in the same gemmule many spicules which exhibit a tendency to 

 pass into a form belonging to those of another genus, thus 



