LARVAL OR SWIMMING STAGES 



A Well Marked Change of State. — In following the development of 

 the oosperm an observer is not met with any remarkably sudden change 

 in structure. Everything depends upon a regular and gradual process of 

 proliferation of cells, a simple, methodical arrangement of these, and a 

 slow, orderly modification to better fit them for their function. The egg, 

 oosperm, segmentation stages, morula, blastula, gastrula pass by such 

 easy and natural transition from one state to the next that it requires 

 close scrutiny to detect in what the difference lies. 



But when the hitherto quiescent organism changes its habit and be- 

 gins to perform automatic swimming movements the phenomenon is 

 sufficiently striking to furnish an easily recognizable and useful mark of 

 distinction between embryonic and larval stages. It denotes the period 

 in time, as well as the condition of structure and activity, when to most 

 people the organism becomes a living animal. There are in the develop- 

 mental life of the oyster only three such clear-cut, radical changes of state 

 — (1) the egg, the smallest, and simplest, free, but non-motile, stage, which 

 springs, as far as all outward appearances are concerned, abruptly from 

 the largest and most completely organized of all the stages, viz., from the 

 fixed, adult mother-oyster; (2) the larva, distinguished as we have just 

 seen by the beginning of locomotion; (3) the spat, even more remarkable 

 in its sudden precipitation out of the water and fixation upon some solid 

 submerged object. 



The larva is the developing oyster during the whole period of the 

 oyster's free-swimming life. It begins with the simple cellular structure 

 of the gastrula and advances until the rudiments of most, if not all, of the 

 tissues and organs of the oyster are originated and brought to some de- 

 gree of efficiency. It swims about, takes in and digests food, grows, and 

 is sensitive to surrounding conditions. 



The Shell-less Larva. — The age at which swimming begins may be 

 considered to be about five hours, reckoned from fertilization, but is sub- 

 ject to variation, depending upon a number of factors, chief of which is 

 temperature. At Shediac (east coast of New Brunswick), July 7, 1909, I 

 put together eggs and sperms of three females and two males in beakers 

 at 10:30 a.m. and at 7:30 p.m. there were plenty of swimming larva. The 

 the temperature of the sea was 17 . 5° C. ( = 63 . 5°F.) and the salinity 1 . 020. 

 At Bay du Vin (southern portion of Miramichi bay), Aug. 5, a similar ex- 

 periment just before 12 (noon) gave an abundance of swimming larvae at 



