570 INVEETEBEATA CHAP. 



the most noticeable are Morgan (1891, 1894), who worked out the 

 later development of several varieties of Tornaria which occur on the 

 east coast of North America ; Eitter (1894), who described stages of the 

 development of a species of Tornaria from the Pacific coast of North 

 America; and Davis (1908), who described the development of 

 a Balanoglossid (Dolichoglossus pusillus), with yolky eggs and 

 shortened development, which appears to be closely allied to the 

 species on which Bateson worked. Finally, a German, Heider (1909), 

 has succeeded, for the first time, in obtaining the fertilized eggs of a 

 Balanoglossid (Balanoglossus clavigerus), which has in its life-cycle a 

 Tornaria larva ; and he reared the eggs until the typical Tornaria 

 larval form had been attained. 



BALANOGLOSSUS 



A fairly complete account of the development of a Balanoglossid 

 can therefore be pieced together by adding Heider's work to that of 

 Morgan. It is probable that the New England Toruaria belongs to 

 some other species of the genus Balanoglossus. We prefer to take 

 the researches of these two workers as the basis for our detailed 

 account rather than to base our account on the development of 

 Dolichoglossus, as worked out by Bateson and Davis, because we hold 

 that a roundabout development, including the formation of a pelagic 

 larva with a long free-swimming existence, is the primitive type of 

 development, and that a shortened development with a larva having 

 a very short free-swimming life, and in which the adult features 

 appear very early, like that of Dolichoglossus, is a secondarily modified 

 condition of affairs. 



Heider obtained the fertilized eggs amongst a consignment of the 

 adults which were collected at the Zoological Station at Trieste and 

 sent to him at Innsbruck. During the journey males and females 

 discharged their genital products and a natural fertilization (see 

 p. 485) resulted. Subsequently, when on their arrival the adults 

 were placed in sea-water with sand at the bottom, they formed burrows 

 in it, and at the mouth of one of the burrows a slimy mass contain- 

 ing thousands of eggs was seen. These eggs, however, were not 

 fertilized. 



The eggs were small and filled with coarse giains of yolk which 

 were uniformly distributed, so that the only way of distinguishing 

 animal and vegetative poles was the nearness of the egg-nucleus to 

 the former. In the fertilized eggs the usual two polar bodies were 

 formed, of which the first divided. 



The eggs segmented with perfect regularity into blastomeres of 

 equal size, and gave rise to perfectly spherical thick-walled blastulae, 

 with comparatively small blastocoeles whose walls were composed of 

 a very large number of narrow, cylindrical, ciliated cells. Heider 

 considered that the development up to this point recalled that of 

 Echinoidea, but the Balanoglossid blastula differs from the Echinoid 



