394 LO WER FORMS OF MARINE LIFE 



Ocean, referable to the family Pharetronidce. When this family was first. 

 established, it was believed to be extinct, with its latest representative, in the 

 Maestricht Chalk of Belgium ; but, inclusive of the Christmas Island species, which 

 constitutes a subfamily by itself, six living genera, each with a single species, are 

 now known. Murrayona differs from the other genera in that the skeleton 

 consists of a firm skeletal network devoid of spicules, and overlain by a superficial 

 layer of scales, the axial core of spicules found in the other living genera having 

 been discarded. The absence of spicules in some of the fossil Pharetronidcs may 

 be due to the same cause, although in other instances it may be the result of 

 fossilisation. 



In conclusion, reference may be made to certain curious masses dredged a 

 few years ago in the Bay of Bengal, which proved to be sponges associated with 

 gregarious molluscs of the gastropod family Vermetidce, the latter being embedded 

 in the former. The masses are of two types, one consisting of shells with serrated 

 ridges embedded in moderately hard black sponges, and the other of smoother 

 shells associated with stony sponges, varying in colour from red to yellow. The 

 ridged shell is Siliquaria muricata, and the associated sponge Spongoscites topsenti. 

 The second type comprises two molluscs, Spiroglyphus cummingi and Siliquaria 

 cochlearis, the associated sponges being two forms of the species known as 

 Racodiscida sceptrellifera, which differ from one another in colour. When fresh, 

 the masses of the second type must have had a brilliant appearance, the sponge 

 being red or orange, the shells pink, and the soft-parts of the molluscs yellow. Both 

 the two sponges associated with the three Vermetidce are found elsewhere growing 

 alone. 



Sponges form the lowest group but one in the whole animal kingdom ; the- 

 lowest of all being the Protozoa, or single-celled organisms, such as the so-called 

 animalcules. In such an enormous group mention of any particular genera or- 

 species is out of the question on this occasion. 



