ZOOLOGY AND BOTANY, MICIiOSCOPY, ETC. 523 



forms being those which live freely between tide-marks, the region 

 which is best suited to the needs of the animals, and which possesses the 

 largest number of species. The organs of the body may be divided 

 into three sets, according to the nature and degree of their variations in 

 the different species. Thus foot, head, mouth-cavity, sub-radular organ, 

 sugar-glands, and most of the central nervous system, are reckoned as 

 stable organs : they vary very little in the different families. Other 

 organs, such as the larval pedal gland, the mucus-producing ridges of 

 the mantle border (SehleimJcrauseii), are to be regarded as regressive, 

 occurring only in the simplest forms. Again, the cuticular structures 

 of the mantle, the mantle itself, the shells, the lateral folds, the gills, 

 the salivary glands, the stomach, liver, and gut, the nephridia, genital 

 organs, heart and circulatory organs, nervous system and sense organs, 

 are progressive organs, varying according to the degree of specialisation 

 attained. A fourth class of organs are those which occur in only a 

 single species or in a genus as adaptations to a special manner of life. 

 Though the Polyplacophora and Aplacophora differ from one another 

 very markedly, yet the resemblances are sufficient to justify the inclusion 

 of both in the class Amphineura. The Solenogastres are to be regarded 

 ■as a side-branch from the Chitonidse stem ; in almost all respects they 

 are more lowly organised than the Chitonidae. The author regards the 

 segmentation of the shell in the Chitons as a consequence of the muscu- 

 lar movements involved in the curling inwards of the body. It has 

 secondarily produced a pseudo-segmentation, which is most distinct in 

 the muscles of the shell, but also affects, to a greater or less extent in 

 the different species, the heart, arteries, and other organs. This 

 pseudo-segmentation is independent of the repetition of the gills, which 

 depends upon the size of the organism and its need for oxygen. The 

 author believes that the only connection between molluscs and annelids 

 is that both are descended from a turbellarian-like ancestor. 



5. Lamellibranchiata. 



Development of Nucula delphinodonta. * — Prof. Gilman A. Drew 

 has studied the development of this American species, in order to com- 

 pare it with that of Yoldia limatula. He finds that, though the embryos 

 resemble each other in most respects, the difference in the size and dis- 

 tribution of their surface cilia makes them appear different. In Nucula 

 the test or surface cells ( = velum) have their cilia uniformly distri- 

 buted, while in Yoldia they are arranged in bands. The embryos of 

 Nucula develop in the protecting brood-sac, and the test would appear 

 to be functionless ; it is shed much earlier than in Yoldia. The apical 

 plate is extensive, but, like the test-cells, carries only short cilia. The 

 foot is very small at the time when the test is shed, and seems never 

 to be used in creeping. The pericardium appears to be schizocoelic in 

 origin. The heart is formed round the intestine, but later becomes free 

 from it and dorsal to it. At the time when the animals become 

 sexually mature, the hypobranchial glands develop. They form the 

 secretion which, when mingled with foreign particles, gives rise to the 

 remarkable external brood-sac. 



m Quart. Journ. Micr. Sci., xliv. (1901) pp. 313-91 (6 pis. and 10 figs.). 



