62 SUMMARY OF CURRENT RESEARCHES RELATING TO 



would seem to be found. Yet again, two kinds of glandular cells are 

 developed in the integument. In addition to the ectodermal nerve- 

 supply, there is an endodermal system of enteric nerves; the oesopha- 

 geal portion supplies only the musculature of the oesophagus, but this 

 is primitively endodermal in origin ; the true ectodermal portion is 

 found in the stomach and intestine, and its fibrils are richly supplied 

 with ganglionic cells, and distinctly separated from the connective- 

 tissue fibres. 



The four regions of the digestive tract — oesophagus, stomach, intes- 

 tine, and rectum — of Synapta, are both histologically and morphologi- 

 cally sharply separated from one another ; and the whole tract is 

 distinguished from that of the pedate Holothurians by the facts that 

 the longitudinal layer of muscles lies externally and not internally 

 to the circular, that there is a special gastric epithelium, and that 

 the internal layer of connective tissue is strongly, while the outer 

 layer is feebly developed. 



The muscles of the body-wall appear to attain a remarkable 

 strength in the Synaptic! se. 



Nervous System of Holothurians- * — E. Semon agrees with Johannes 

 Miiller in regarding the radial nerves as the primary, and the oeosopha- 

 geal ring as the secondary portion of the Holothurian nervous system ; 

 the histological characters of the former are more complicated, and 

 they appear to be developed before the latter. In the young Synaptas 

 from which careful transverse sections were made, the radial nerves 

 were seen to be completely developed and to have at either pole a 

 relatively large swelling, but there was no indication of any commis- 

 sure. The author disagrees with the ordinarily received doctrine that 

 the nerves end in a point near the anus, for in that region he has been 

 able to detect a considerable thickening in the nervous band, and he 

 inclines to believe that a secondary anal commissure is developed ; on 

 the other hand, in the Aspido- and Dendrochirota there is certainly 

 no such commissure, the appearance of which is really due to the 

 elastic connective-tissue-fibres there developed. At either end of 

 the digestive tract special sensory regions appear to be present, and 

 Holothurians are notoriously sensitive at their mouth and anus, 

 possibly to protect themselves from the parasites by which they are 

 often infested. 



After some observations on the topographical relations of the 

 nervous system, the author passes to the histology ; the difficulty of 

 the investigation is even greater than in other groups of animals, 

 owing to the small size of the elements, and our slight knowledge of 

 the minute structure of the other organs. After the animal has been 

 killed in boiled sea water, whereby it dies in an extended condition, 

 and the elements have been isolated, it is as well to wash them several 

 times in distilled water, as they have always a quantity of by-products 

 associated with them. Staining reagents must be very carefully applied 

 to the fresh tissues, as, unless they are very dilute, they will soon 

 blacken the whole tissue. The author found his greatest difficulty in 



* Jenaisch. Zeitschr. f. Med. u. Naturwiss., xvi. (1883) pp. 578-600 (2 pis.). 



