628 SUMMARY OF CURRENT RESEARCHES RELATING TO 



two distinct lamellae ; the outer is dense, of equal thickness throughout, 

 and remarkably refringent ; reagents bring out in it the appearance 

 of fine undulating fibrils ; the inner layer is loose, with wide fenestra;, 

 and contains numerous cells and brown or yellow pigment-granules ; 

 it is in the latter layer that the blood-vessels ramify ; in the non- 

 vascular parts it is thicker, except on the rectum. The alimentary 

 canal is provided with glands of two different types, viz. : — (1) 

 Mucus-cells ; large, oval, very numerous, placed among the epithelial 

 cells ; they occur in the second curvature, especially at its vascular 

 parts. (2) Glands proper ; they are pyriform and multicellular, the 

 component cells radiating from the centre of the gland ; each gland 

 opens separately into the intestine, and lies in the loose inner layer 

 of the connective tissue, but only in the region between the first 

 opening of the siphon and the end of the oesophagus. The nerve- 

 ring surrounding the mouth is quite distinct from the blood-vessels, 

 and is not embraced, as has been stated, by one of the latter. A 

 further examination of the " heart " confirms the view of its glan- 

 dular nature ; it is broken up by trabecule into a number of cavities, 

 containing cellular elements of two kinds, viz. either regular cells 

 with distinct outline and tolerably refringent protoplasm, or cells 

 with very irregular and indistinct outlines, with transparent but 

 scanty protoplasm and granular nuclei, often two or three in number 

 in the same cell; with these elements occur also brown or yellow 

 granules, some of the yellow ones being aggregated into strawberry- 

 like masses. Injection of the heart by way of the sand-canal shows 

 that it communicates both with the madreporic plate and with the 

 spaces of the connective tissue, and of the body generally ; thus the 

 blood probably undergoes some modification in the organ, and is then 

 in part carried into the general system and in part thrown off from 

 the body. 



In Echinocardium flavescens the internal marginal vessel and the 

 siphon are a little longer than in Sjpatangus, and they terminate at the 

 point of junction between the second and third bends of the alimen- 

 tary canal. This form also differs from Spatangus in possessing a 

 small diverticulum or faecal reservoir in the rectum. 



Development of Asterina gibbosa.* — Professor H. Ludwig finds, 

 as a general result of his important studies, that we have throughout 

 the Echinodermata a mode of development which must be spoken 

 of as a metamorphosis ; the ground- form of the larva is an organism 

 ciliated over the whole surface, with a mouth and anus on one side. 

 By adaptation to different conditions of existence this ground-form 

 has become variously modified as Bipinnaria, Pluteus, &c, which may 

 be known as the secondary larval forms. The mature Echinoderm- 

 form may arise either directly from the primary larva, or from a 

 secondary larval form ; or, further, one secondary larval may be 

 followed by another larval form (e. g. Bipinnaria, Brachiolaria) ; but 

 this interpolation of intermediate stages makes no essential difference 

 in the course of the development. The processes by which the 



* Zeitschr. f. wiss. Zool., xxsvii. (1882) pp. 1-08 (8 pis.), 



