410 HEXRY C. TRACY. 



furnishes favorable conditions for an interchange of substances 

 between the sea water and the blood of the larvae and probably 

 acts as a respiratory organ before the branchial mechanism 

 becomes functional. It is, therefore, comparatively easy for 

 purposes of experiment to change certain chemical characteristics 

 of the body fluids of the larva by altering the character of the 

 sea water around it. 



Opsanus tan (toad fish) is an inactive bottom fish which 

 inhabits shallow, sheltered water. The eggs are found in such 

 localities, fertilized and in the process of development, attached 

 to the under-side of sticks, stones, tin cans, etc. The objects to 

 which the eggs are attached may be brought into the laboratory 

 where they develop in an apparently normal manner if furnished 

 with a constant supply of fresh sea water. The eggs hatch in 

 3 or 4 weeks; the yolk sac, however, remains attached; about 

 three weeks later, the larva has absorbed nearly all the yolk; 

 it then becomes loosened from its attachment and swims free. 



In this species the first observable movement is the heart 

 beat which begins in specimens of 12 to 14 somites. The earliest 

 stage at which muscular movements of the body were observed 

 w r as in a specimen about 19 somites. Movements appear in 

 the first few somites (2 to 4 or 5) ; the somites contract slowly 

 and apparently simultaneously, causing a lateral bending of the 

 anterior body region. 



These movements take place singly and at irregular intervals, 

 often with several minutes between and with no apparent 

 relation between successive movements. Responses to external 

 stimuli, however, do not take place until a much later period, 

 in fact, not until after hatching. There seems, therefore, no 

 question that these early movements are the result of changes 

 in the internal conditions. 



Soon, however, there appears a slight tendency for movements 

 to appear in groups; this tendency increases until at the time 

 of hatching, in addition to occasional single bendings of the body, 

 right and left alternating coils of the body often take place very 

 rapidly for a brief interval, producing a kind of vibratory or 

 "fluttering" movement of the whole body. These "fluttering" 

 movements become more and more predominant during the 

 larval stage. When the free swimming period begins the "flut- 



