ZOOLOGY AND BOTANY, MICROSCOPY, ETC. GG.~. 



the animal plankton there is an especial rarity of Crustacea, Eadiolaria 

 being relatively much more abundant. The vegetable plankton has a 

 •double effect in the purification of the water. It consumes substances 

 •which would otherwise corrupt the water, and it gives out the oxygen 

 which is required by the animal life. 



Plankton of Mammoth Cave.* — Mr. C. A. Kofoid has studied tow- 

 net collections made in the Echo Kiver in the Mammoth Cave, but finds 

 that the organisms of the collections are chiefly surface forms swept in 

 by means of swallow-holes. Oopepods are numerous, but there is no 

 evidence that any of them are true cave-forms ; such cave-forms are in- 

 deed remarkable by their absence. 



Mollusca. 



a. Cephalopoda. 



Development of Placenticeras.j — Prof. J. P. Smith has studied the 

 tlevelopment and phylogeny of this type, and his results, he says, illus- 

 trate the possibility of deciphering racial history in individual ontogeny. 

 The stages wholly lost out of the ontogeny lie between the nautiliau 

 23iotoconch and the glyphioceran larval stage, perhaps corresponding 

 to the period before hatching. All later stages are recorded in the 

 ontogeny with a fair degree of distinctness. The protoconch cannot 

 be correlated with any nautiloid, but the later stages can be 

 compared with ammonoid genera, the exactness of the correlation 

 becoming less as the stage advances, on account of unequal acceleration 

 of development of ancestral characters, but on the other hand easier, 

 on account of the greater number of characters one has to deal with. 

 The earliest larval stage is nautiloid in septa, but ammonoid in its 

 calcareous protoconch. The middle larval stage is comparable to the 

 Palaeozoic GJypldoceras, the last larval stage to the Mesozoic Mannites. 

 In the adolescent period Placenticcras goes through a Cymbites-likc 

 stage, an aegoceran stage, a perisphinctoid stage, is then like Cosmoceras 

 of the Jurassic, and finally resembles Hoplites of the Cretaceous. It is 

 thus shown by ontogenetic study that Placenticeras developed out of 

 Hoplites, and comes near the Stephanoceratidae. The development gives 

 an unusually fine illustration of the law of acceleration or tachygenesis, 

 with its two corollaries, unequal acceleration and retardation. 



y. G-astropoda. 



Minute Structure of Snail's Stomach.^— Puna Monti describes the 

 lining epithelium which consists of (a) caliciform cells full of mucus, 

 und (b) cylindrical cells with cilia. The tissue which supports the 

 epithelium includes large connective-cells (Leydig's), fibrillar connective- 

 tissue, pigment-cells, smooth muscle-fibres, and vessels lined with endo- 

 thelium. Outside this there are longitudinal and then circular muscles ; 

 und this muscular sheath is enveloped in a connective- tissue rich in large 

 cells analogous to those of the sub-mucosa. 



* Trans. Arner. Micr. Soc, xxi. (1899) pp. 113-2o\ 



t Contributions to Biol, from the Hopkins Seaside Lab., xxii. Reprint from Proc. 

 * 'ulifornia Acad. Sci.,i. (1900) pp. 181-241) (5 pis.). 

 % Rend. 1st. Lombardo, xxxii. (1S99) pp. 108G-97. 



