64 COMMISSION OF CONSERVATION 



in structure as in the full-grown larva. In a spat of -44 mm., in which 

 the foot was cut longitudinally, it is shrunken until the lateral walls of the 

 distal half are near together and the long cilia stand out in shaggy clusters. 

 In one of -79 mm. (Plate VI, figs. 27, 28), it is reduced to a small, ventrally 

 grooved mass, below and behind the mouth and between the bases of the 

 inner palps, in which condition it may be traced until it gradually flattens 

 out and thins into the integument of this region. 



Both velum and foot are functionally at their best at the time of 

 fixation. Immediately after this they rapidly dwindle until they have 

 completely disappeared, not even leaving a trace of any of their contained 

 organs, such as supra-cesophageal and pedal ganglia, otocysts, and byssus 

 gland. 



Method of Fixation: Byssus Gland. — Fixation takes place at the end 

 of larval life, and is the one thing which distinguishes the late stages of the 

 full-grown, free-swimming or free-creeping, well organized larva from the 

 young stages of the similarly constituted but attached spat. Any of the 

 soft parts, as velum, mantle, or foot, have a tendency to cling to objects 

 against which they press, and larvae of all stages are liable to be found 

 temporarily resting in such a condition. But this is not permanent, and 

 is not what is meant by fixation in this work. The soldering of the left 

 valve of the shell fast to a solid object of support is a process for which 

 preparation has had to be made in the organism — preparation not only 

 for the material used but for the ability to apply it. I had formerly sup- 

 posed that fixation was accomplished in an easy and natural way, and 

 almost as a matter of course, by the deposit of the new calcareous material 

 of the first growth of the spat shell at the edge of the left valve of the larval 

 shell, and uniting it at the same time with the object upon which it rested. 



In the preparation of sections it is necessary to decalcify the shells 

 of the specimens before they can be cut. Sections of young spat 

 show on the outside of the left valve a layer which does not occur 

 on the right valve, and which is of a different appearance from 

 the matrix of the calcareous shell. It is somewhat homogeneous or 

 faintly striate and has rather a horn colour, with some tendency to receive 

 staining matter, while in the spat shell the calcareous matter is dissolved 

 out leaving only the thin outline of the periostracum or other organic 

 matter. In my youngest spat of very recent fixation (Plate VI, fig. 10, c) 

 this layer has a thickness of -055 mm., while the remnant of the shell else- 

 where is an irregular line not more than one-eighth as thick. This spat 

 was caught on a strip of window glass and afterwards broken away clean, 

 leaving no chance for carrying away any part of the substratum. The 

 fixation material had been spread over rather more than half of the 

 left valve of the larval shell, beginning almost at the anterior edge, 

 extending almost to the umbo, but falling short posteriorly. On these 



