l60 MARINE BIOLOGICAL LABORATORY. 



a free larval form of its own (whether primary or sec- 

 ondary is immaterial), and as the group arose, through 

 divergence of the adult animals, the larval form was 

 itself affected by the new characters of the adult, and so 

 changed into what I shall speak of as a transfomned 

 larva, or, to put it differently, we may suppose the 

 newly acquired characters of the adult to be thrown 

 back upon the larval form already present. This is 

 shown by diagram C, Fig. 4, when a is changed gradu- 

 ally to ay, az, as the adult X changes into Fand Z. 



There may be many modifications of these three con- 

 ditions I have sketched, but for our present purposes 

 these will suffice. 



To restate the problem before us : we wish to find 

 out, if possible, in which of these ways the larval form 

 in the Pycnogonids has been evolved. Professor Dohrn 

 believes, and his theory is the natural outcome of the 

 position he has taken as to the origin of the group of 

 the sea-spiders, that the larva of the Pantopoda is a 

 transformed larva, and therefore the explanation of its 

 appearance within the group to be represented by 

 Fig. 4, C. Dr. Hoek, on the other hand, believes the 

 embryo to be a primary larval form, and therefore its 

 presence to be accounted for in some such way as repre- 

 sented by Fig. 4, A. Professor Dohrn further believes 

 that the Trochophore of Annelids represents the first 

 larval form which became modified into the Pantopod- 

 larva ; while Dr. Hoek believed that the Pantopod-larva 

 of the Sea-spiders, the Trochophore of the Annelids, 

 and the Nauplius of the Crustacea, have each given 

 rise to their respective groups, and to each represent 

 a primary larval form ; and moreover, he thought these 



