PHYLOGENY OF THE PELECYPODA. 305 



We will now consider an oyster, PI. xxiv, fig. 4, considerably older than our last fig- 

 ure. Ryder (63) published a figure and descrijition of an oyster somewliat yonuger 

 than this and yet older than my PI. xxiv, fig. 3. He compared the anatomy of his fig- 

 nre with that of Professor Huxley's figure (my fig. 25, p. 300) and demonstrated serial 

 connections between these and the adult. PI. xxiv, fig. 4, illustrates a young oyster of 

 the spat stage growing on a glass slide, on August 6, 1888. The single adductor muscle 

 occupies a sub-central position similar to that of the adult. The palps have revolved 

 dorsally and assume a position intermediate between that shown in PI. xxiv, fig. 3, and 

 that characteristic of the adult, PL xxv, fig. 12. They differ from the adult in that the 

 outer pair of palps forms a hood above the mouth which opens downwai'ds and not as 

 directly forwards as in the adult. Both these differences are discussed by Ryder (63). 

 Four palps exist, but they are marked by few (five) furrows on their surface, instead of 

 a great number as in the adult. Ryder observes this difference, and says that though 

 few in the young he has counted over a hundred furrows on one side of the lower palp 

 of an adult. 



The heart is situated in a pericardial cavity above the adductor muscle, as in adults. 

 From ninety to one hundred and ten heart-beats per minute were counted. This is a very 

 high number, as Pi'ofessoi- Ryder (64) notes that in the adult the heart-beats probably 

 do not exceed twenty per minute. A large pulsating artery, v, passes to the mantle 

 boi-der where it is confluent with the posterior tips of the gills, and lesser i-amifying ar- 

 teries bifurcating frequently are spread over the mantle in every direction. These are not 

 shown in the figure as they would confuse other details. The blood corpuscles travers- 

 ing the arteries may be readily seen through the thin shell as indicated by arrows and, 

 when the heart is beating rapidly, the blood may be seen returning to the heart through 

 the canals on the dorsal border of the gills. It passes out from these canals through the 

 foramina c and fZ, and also from between foramen d and a point neai'er the anterior tips 

 of gills. At d the afferent and efferent vessels are connected and though most of the 

 blood from the gills passes into the heart, some corpuscles may be seen joining the ef- 

 ferent current and flowing into the large artery, v, which supplies the mantle border. 



This specimen growing on glass aflibrded excellent opportunity to study the gills. 

 The filaments of the two gills. A, A,' Pi. xxiv, fig. 4, are connected by several rows of 

 cross bais, instead of only a single row on the ventral margin. (Compare with PL 

 XXIV, fig. 5.) An oyster, 6 mm. high, was observed to have five cross flhxments in the 

 larger gills. The two first gills originated simultaneously as far as known. They be- 

 gan as tubular disconnected bags, as described; by the concrescence of lateral fleshy pro- 

 cesses and of the inwardly gi-own recurved tips of the filaments, their pi-esent form, which 

 is diagrammatically shown in PL xxiv, fig. 8, has been built up. But this accounts for 

 only two gills and there are four in the adult; the development of the last two gills has 

 not been previously studied, though R}' der (63) states that in very young spat only two 

 are present. When first studied on the sixth of August, the young oyster, PL xxiv, 

 fig. 4, besides the two larger gills, A and A', had a third very short gill, B', on the right 

 side; but no corresponding, fourth short gill on the left side. Tiie gill B' was continu- 

 ous along the margin of the adjoining gill A' for its whole extent. It originated from 

 the same base as did A' for the outer border of ^4' was first formed, the inni'r border be- 



