168 THE MICROSCOPIST. 



a long stem to secrete calcareous matter, not merely exter- 

 nally, but in the substance of its body and tentacles, such 

 polyp when dried would present some such appearance as 

 the fossil Encrinoid Echinoderms of past times. The im- 

 agination of such a polyp without a stem, and having 

 sucker-like disks on its arms, will give us the picture of 

 a star-fish (Asterias). Imagine the rays diminished and 

 the central part extended, either flat or globular, and we 

 have the form of Echini with the spines removed. The 

 Holothurice have elongated membranous bodies, with im- 

 bedded spiculae. 



The structure of Echinoderms is quite complex, and 

 belongs to comparative anatomy rather than microscopy, 

 yet some directions for the study of these forms is essen- 

 tial to our plan. 



Thin sections of the shells, spines, etc., may be made by 

 first cutting with a fine saw, and rubbing down with a 

 flat file. They should be smoothed by rubbing on a hone 

 with water, cemented to a glass slip with balsam, and 

 carefully ground down to the required thickness. They 

 may be mounted in fluid balsam. 



Many Echinoderms have a sort of internal skeleton 

 formed of detached plates or spiculse. The membranous 

 integument of the Holothuriae have imbedded calcareous 

 plates with a reticulated structure, and they are often 

 furnished with appendages, as prickles, spines, hooks, etc., 

 which form beautiful microscopic objects. 



The larva of an Echinoderm is a peculiar zooid, which 

 develops by a sort of internal gemmation. One of the 

 most remarkable of these larvae has been called Bipin- 

 naria. 



IX. BRYOZOA OR POLYZOA. Microscopic research has 

 removed this class from the polyps, which they resemble, 

 to the molluscan sub-kingdom. They have a group of 

 ciliated tentacles round the mouth, but have a digestive 

 system far more complex than polyps. They form delicate 



