245 



It is evident that the amount of yolk substance contained in the egg 

 has an important bearing on the length of time required for the develop- 

 ment. When food enough is contained in the egg for sustaining the embryo 

 until metamorphosis is completed, the self-feeding larval stage is done 

 away with as unnecessary and the development thus considerably short- 

 ened, e. g. Heliocidaris erythrogramma, Peronella Lesueuri, Ophionereis squa- 

 mulosa and many other forms with abbreviated development, not mentioned 

 in the present memoir. That also temperature has an important bearing 

 on the time required for the development is an established fact; interest- 

 ing results are sure to be obtained on studying the development of widely 

 distributed forms in various places, where conditions are different (tropical, 

 extratropical). The few facts known, e. g. of the development of Peronella 

 Lesueuri in tropical seas, as compared with the observations giveni above, 

 tend to show that the development proceeds at a conspicuously quicker 

 rate at the higher temperature of the tropics than in the cooler climate of 

 extratropical regions. 



The said factors, yolk and temperature, cannot, however, account for all 

 the differences. Thus e. g. of Tripneustes esculentus and Lytechinus varie- 

 gatus, both living in quite shallow water in the tropics and both having 

 small eggs, poor in yolk substance, the former takes three weeks to reach 

 the stage of beginning metamorphosis, the latter only 13 days; or En- 

 cope micropora assuming the shape of a young Pluteus already at the age 

 of 12 hours, while Mellita 6-perforata, living under similar conditions re- 

 quires the double time for reaching that stage. But altogether too little 

 is known as yet for giving a reasonable base for an attempt to find out the 

 causes of these differences. The observations recorded here may only 

 serve to prove that here is a problem worth studying. 



In a very interesting paper on "Sea-temperature, breeding and distribu- 

 tion in marine animals"^) I. H. Orton comes to the result that "in those 

 parts of the sea where temperature conditions are constant or nearly con- 

 stant, and where biological conditions do not vary much, marine animals 

 will breed continuously." He concludes that this will be the case in the 

 tropics, founding on the statement of Semper ("Animal life", p. 110) that 

 in the Philippines he could not detect a single species (of Invertebrates) of 

 which he could not "at all seasons find fully grown specimens, young ones 

 and freshly deposited eggs." This phenomenon, Orton states, "appears 

 to be generally recognized for the tropics, but it would appear that definite 

 systematic work on the breeding and rate of growth throughout the year 



1) Journ. Mar. Biol. Assoc. United Kingdom. XII. 1920'. 



