A. Anatomy, Physiology, Embryology. III. Asteroidea. 189 



more or less completely absorbed into the basal cup. The deltoid plates of the 

 Blastoids are calyx-interradials and not homologous with the orals ofNeocrinoids. 



III. Asteroidea. 



See also Metschnikoff, supra, p 185, and Niemiec, infra, p 192. 

 Hamann finds muscular layers, both of circular and of longitudinal fibres in 

 the body- wall of an Asterid, the latter being uniformly distributed, and not re- 

 duced to five special bands. Tracts of nerve fibrils run among the basal prolon- 

 gations of the cells in the dorsal epithelium, some of which are sensory cells con- 

 nected with the fibrils. The greater part of the fibrils of the radial nerves and of 

 their connecting Gehirnring occupy a similar position in the thickened ambulacral 

 epithelium. Small ganglion cells are intercalated among them, and a larger kind 

 occurs in the terminal tentacle together with especially developed sensory cells. 

 Longitudinal nerve fibrils extend far down into the alimentary canal from a fold 

 in the lip. The histology of the tube feet in 4 species is described minutely [see 

 Bericht for 1883 I p 129]. The gills are processes of the dorsal body-wall in 

 which all its layers take part. It contains a system of cavities which Greeffcalled 

 a canal system and Ludwig supposed to be derived from the enterocoel. Hainaun 

 finds it to be a true schizocoel arising as clefts in the meseuchym. The so called 

 perihaemal canals are of the same nature together with their lateral extensions 

 between the tube feet, which join the dorsal schizocoel spaces. They are not lined 

 by a true epithelium like that of the enterocoel, but only by flattened connective 

 tissue cells, groups of which were regarded by Lange as ganglionic masses. Both 

 dorsal and ventral schizocoel spaces open into the restricted tubular portion of the 

 enterocoel which contains the so called Heart . The so called radial blood- 

 vessel within the vertical septum of the ventral schizocoel space is not a con- 

 tinuous tube, but a series of lacunae arising in the connective tissue. The anal 

 ring, the genital and the visceral vessels are also lacuuar in structure. The former 

 is connected with the Chromatogen organ (miscalled Heart] parts of which pro- 

 ject into the enterocoel beyond the tubular space enclosing the water tube, so that 

 the walls of this space are incomplete in places. The Chromatogen organ 

 does not open externally through the madreporite, but terminates blindly as de- 

 scribed by Ludwig. It consists of a number of anastomosing canals, mostly longi- 

 tudinal, and lined by strongly pigmented large cells, some of which may become 

 detached. The walls pass directly into those of both dorsal and ventral blood- 

 lacunae. On the central side of the aual ring is another ring canal connected with 

 two genital canals and two more opening into the Chromatogen organ . All these form 

 an excretory system lined by similar cells to those of the glandular Chromato- 

 gen organ. The various parts of the digestive tube are sharply distinguished 

 histologically. The subepithelial nervous layer extends from the lip far down into 

 the mid-gut. Its walls and those of the hepatic coeca are ciliated internally and 

 contain large flask-shaped unicellular glands ; those of the stomach secrete a poi- 

 sonous fluid used in killing the shellfish that serve as food, and those of the hepa- 

 tic coeca secrete a digestive juice. From the part of the Chromatogen organ which 

 projects freely into the enterocoel there arises a vascular plexus, which spreads 

 over the dorsal surface of the stomach as a system of intercommunicating lacunae 

 in the connective tissue. The pore-canals of the madreporite lead nowhere but 

 into the water-tube (stone-canal) and its ampullae, just as described by Lud- 

 wig. The epithelium lining the coiled parts of the tube is lower than that found 

 elsewhere and has smaller cilia. These perhaps work the opposite way to the 

 larger ones so as to produce two streams, an incurrent and an excurrent one. 



