8 STUDIES IN BIOLOGY. 



and mesosoma a pair of lamellar organs, the gills (Fig. 1 ; and i g and 

 o g, Fig. 35) . There are two gills on each side, and each gill-plate 

 is doubled upon itself, so that we can distinguish an inner and 

 an outer lamella for each gill. The two gills arise from the 

 same line, and, taking the animal in its proper position, the first 

 lamella of each gill is called the descending lamella as it hangs 

 downward (Figs. 35 and 44). The inner gill is doubled upwards 

 towards the mesosoma, and the outer gill is doubled upwards towards 

 the mantle or shell. The plates or lamellae thus turned upward are 

 called the " ascending lamellae." These ascending lamellae in M. latus 

 become attached at their upper side to the mantle or to the body 

 (Fig. 35) . By reference to M . edulis, in which the ascending lamellae 

 remain free (Fig. 37), the structure of the gills can easily be ex- 

 plained. From this figure it is seen that the outer lamella of 

 the outer gill and the inner lamella of the inner gill are the ascend- 

 ing lamellae which are free in M. edulis and magellanicus ; while the 

 inner lamella of the outer gill and the outer lamella of the inner 

 gill are the descending or the original lamellae of each gill. 



Between the two lamellae of the inner gill in M . latus, or by the 

 inner side of the descending lamella of the inner gill in M. edulis, 

 near the hinder end of the mesosoma on each side, is a small papilla 

 {g p, Fig, 1), the genital papilla. Just behind this papilla is an 

 opening, the renal aperture (r a, Fig. 1). 



On the upper side of the posterior adductor in M. latus or on the 

 posterior side in M. edulis is the anus, the latter part of the rectum 

 being prominent upon the muscle. 



Internal Structure. 



A great part of the body is occupied by the muscles upon which I 

 have already touched in speaking of the impressions on the shell. 

 The posterior adductor is a large transverse bundle of muscle-fibres 

 extending from valve to valve. The retractors are muscular bands 

 extending from^their respective impressions to the base of the foot 

 and byssus (Fig. 1). Above and supported by these muscles lies the 

 main mass of the viscera. The space above the anterior retractors is 

 occupied chiefly by the stomach and the surrounding liver. Above 

 the posterior retractors is the pericardium, containing the two auricles 

 and the ventricle, with part of the rectum which pierces the ventricle 

 (Fig. 1). Below the pericardium are parts of the intestine and 

 pyloric caecum. The anterior coil of the intestine passes to the left 

 of the stomach. The pyloric caecum extends into the left mantle- 

 lobe (Fig. ] ; and p c, Figs. 13 and 14). (A more detailed account 

 will be given under the name of each organ.) 



