228 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tlio bvssus apparatus almost always represented, he is inclined to 

 regard it as a characteristic organ of the Lamellibranch tyi^c. 



Ho has been led to think that the so-called pori aquiferi are 

 nothing more than the orifices of degraded byssogcnous glands. If 

 water does really ent^r the circulatory system, Barrels is of opinion 

 that it must do so by fine intercellular canals, or by cndosmosis, or 

 some other way, but not by the "pori aquifori." 



Nervous System of Embryos of Limacina, and the Relations 

 of the Otocyst.* — S. Jourdain finds that in the embryos of Limacina 

 the peri-ajsophageal ganglionic masses differ considerably from those 

 of tlie adult ; the posterior post-cesojihageal ganglia, which are 

 markedly asymmetrical, are feebly developed, and widely separated 

 from one another ; the intermediate ganglia have distinct connectives, 

 and the anterior are still connected by two commissural bands. In 

 the adult there is a marked concrescence of all these parts. The 

 auditory nerve arises from a small ganglionic nucleus, placed on the 

 course of the connection which unites the intermediate mass of the 

 post-oesophageal group of ganglia with the jjedal ganglion ; and the 

 otocyst is at fiii-st provided with a stalk, which is formed by the 

 auditory nerve. The author disagrees with the view of Lacaze- 

 Duthiers that, whatever be the position of the otocyst, it is always 

 innervated by the cerebroid ganglia. Jourdain docs not regard the 

 otocyst as a true auditory organ, but as an ap2)aratus by means of 

 which the animal becomes acquainted with the least changes in tho 

 nature of the surface to which its foot is applied, or in the water in 

 which it moves. 



Shells of MoUuscs.t — In 1858 G. Rose explained the diflferent 

 powers of resistance observed in shells of molluscs within the same 

 geological stratum by a theory that the parts that resisted best were 

 composed of calcite, and the least resistcnt of aragonite. The 

 external " prismatic " layer of a typical shell alone contained calcite, 

 the " mother-of-jjcarl " and "porcelain" layers showing aragonite. 

 Two axes of double refraction were seen in the " mother-of-pearl " 

 layer, and the theory was proved by careful observations of hardness 

 and specific weight. 



Herr v. Gumbel has recently investigated the subject, and finds that 

 the data for determining the question by specific weights are useless, 

 because of the presence (1) of animal matter up to 1"5 per cent, in 

 proportions varying in the different layers, and (2) of foreign and 

 non-calcic matter. The typical shell consists (1) of an outer " honey- 

 comb" layer, so called because calcification has taken place in vertical, 

 hexagonal cells, largely subdivided, formed by a stout membrane ; 

 (2) of a " mother-of-pearl " layer made up of numerous parallel hori- 

 zontal membranes, vertically united at intervals by connections of an 

 organic nature with the small intervals between their parallel lamellaa 

 filled with tho calcifying substance ; and (3) of an " ivory " layer 



* Comptea EencUis, c. (1885) pp. 383-5. 



t Zt'itschr. Deutsch. Geol. (jiesell., xxvi. (1884) p. 386. See Naturforscher, 

 xviii. (1885) pp. 15-7. 



