64 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Histology of Heteropods.* — Erich Reupsch has made a detailed 

 study of Pterotrachea coronata, and gives an account of the minute 

 structure of the integument, the supporting substance, the muscles, the 

 alimentary system, the gonads, the so-called " nucleus," the excretory 

 organ, the respiratory and vascular systems, and the nervous systems. 



5. Lamellibranchiata. 



Minute Structure of Shell of Anodonta cellensis.t — R. Rassbach 

 has made a detailed study of this. The periostracum-substance is 

 formed from the epithelial cells of the mantle-margin-fold and from 

 a portion of the associated epithelial cells of the outer surface of the 

 mantle. The organic substance of the external unelastic ligament 

 arises as a product of the mantle-seam-cells, which lie below the post- 

 nympheal groove. The whole of the external epithelium of the mantle 

 is in a position to form organic substance with the properties of normal 

 periostracum. 



In special cases, the whole of the external epithelium of the mantle 

 may form a prismatic layer, after the formation of an organic foundation 

 of periostracum substance. 



A sort of focus of each prism is formed by a yellow or brown 

 roundish body, which seems to be composed of small periostracum-like 

 particles. 



The nacreous layer, which is usually embedded on the prismatic 

 layer, serves especially for strengthening the shell. It is for the most 

 part formed by the epithelium of the mantle surface. 



A special layer is the " clear layer." It arises usually at the muscle- 

 insertions, and independently of the other layers. In this it differs 

 from the prismatic layer, which can only be formed on an organic 

 foundation of periostracum substance. 



The more prominent and distant Hnes of growth on the surface of 

 the shell are annual rings, and mark the close of a growing period. Of 

 similar significance are the parallel lines, at the posterior end of the 

 ligament, running across the external surface of the ligament from one 

 valve to another. 



All portions of the shell, shell-margin and shell-centre, muscle- 

 insertions and ligament, may be regenerated after injury. The re- 

 generated portions always show the various layers and are formed from 

 the adjacent epithelium. 



Arthropoda. 

 «. Insecta. 



Integumentary Sense-organs of Dytiscus marginalis.t— R.^Hoch- 



reuther distinguishes sensory hairs, sensory bristles, sensory rods, tactile 

 and gustatory papillae, pitted cones (solid and hollow), vase-shaped 

 organs, and cupola-shaped organs. A mechanical sense is locaUzed in 

 sensory hairs, sensory bristles, sensory rods, tactile papillae, solid pitted 



* Zeitschr. wiss. Zool., cii. (1912) pp. 249-376 (8 pis. and 31 figs.), 

 t Zeitschr. wiss. ZooL, ciii. (1912) pp. 363-48 ^64 figs.). 

 i Zeitschr. wiss. ZooL, c. (1912) pp. 1-114 (102 figs.). 



