ORGANS OF THE SPAT 71 



by retaining its connection with the body by a septum carrying blood vessels and 

 nerves. 



Ryder ('82, '84, p. 787) wrote: "One of the most conspicuous differences between 

 the symmetrical larva and the yotmg spat is the absence of gills in the former and their 

 presence in the latter." 



Jackson ('90, p. 301, 303-306) knew the gills from the youngest stage of the spat, 

 but appears to have had only few specimens. Although particularly interested in 

 phylogeny he made important observations in embryology. The specimen (his Plate 

 XXIV, fig. 4) in which he first recognized the right outer hemibranch was about 5 mm. 

 in height, and must have also had a corresponding left outer one, although it could not 

 be seen. As already stated, all four hemibranchs can be recognized in sections of spat of 

 3 mm. height. .Jackson states that the "ends of the gill-filaments in the spat. Fig. 3, 

 are recurved and joined by concrescence at their tips with the recurved filament tips of 

 the opposed gill-lamella." This is undoubtedly a mistake but one easily made in view- 

 ing the specimen from the surface. At the period of Fig. 3, which was a spat of approxi- 

 mately double the height of the prodissoconch, the splitting of the filaments of the left 

 gill would be so complete that it would be an easy matter to follow dowm one half of a 

 filament and up the other as if it were really reflected. Long before I had prepared 

 sections I had done this but never was quite satisfied at not finding any free recurved 

 tips in the act of becoming reflected as required by theory. 



The Intestinal System has all its parts represented in the larva. With 

 the growth of the spat these suffer certain alterations in relative sizes, 

 shapes, and positions. Perhaps the most radical change is produced by 

 a rotation of the body in such a way that the mouth moves forwards and 

 upwards towards the antero-dorsal margin of the prodissoconch, where 

 the anterior adductor muscle used to be. This rotation accompanies and 

 is associated with the loss of the velum, which in the larva is so large as to 

 occupy all the fore-part of the cavity of the shell, forcing the mouth and 

 oesophagus backwards to near the median frontal plane of the body, and 

 causing the oesophagus to be curved backwards around it. In spat soon 

 after fixation, when the velum is completely cleared away, the mouth can 

 occupy its normal position as in the adult. Such a rotation may appear, 

 at first thought, inexplicable, but, when it is remembered that the body 

 of the larva is possessed of great freedom of movement, being at times 

 thrust forwards, putting the retractor muscles on the stretch, it can 

 be readily understood. In fact it is conceivable that these muscles may 

 be made to do duty in bringing about the rotation and in fixing the body 

 in its new position, for after the loss of velum and foot, there is no longer 

 any need of such free movement, consequently these muscles may lag 

 behind other parts in growth, exerting a tension as they do so sufficient 

 to cause the rotation. In harmony with the view of a rotation of the 

 body is the upward movement of the anterior adductor muscle and a dovvai- 

 ward movement of the posterior adductor muscle, the forward curving of 

 the oesophagus and the more rapid growth of the posterior lower parts of 

 the body. 



Mouth and Palps. — The mouth persists from the larva, but, with the 

 taking of its new position and the change in the method of collecting food, 

 there is an attempt at enlarging its capacity. This is partly accomplished 

 by lateral extension, but also by development of palps. 



