258 



ARTICULATA. 



equivalent to the segments of the Chffitopoda. The alimentary tract 

 is not continued into the hindermost of them. 



In Thecidium the ova are very large, and development takes 

 place in a special incubatory pouch in the ventral valve. The 



embryos are attached by suspenders to the 

 * two cirri of the arms which immediately ad- 



join the mouth. There is a nearly regular 

 segmentation, and a very small segmentation 

 cavity is developed. There is no invagina- 

 tion ; but cells are budded off from the walls 

 of the blastosphere, which soon form a solid 

 central mass, enclosed by an external layer 

 the epiblast. In this central mass three ca- 

 vities are developed, which constitute the me- 

 senteron and the two halves of the body 

 cavity. Around these cavities distinct walls 

 become differentiated. The body (Lacaze 

 Duthiers, No. 327) soon after becomes divided 

 into two segments, of which the posterior is 

 the smaller. The hinder part of the large 

 anterior segment next becomes constricted off 

 as a fresh segment, and subsequently the re- 

 maining part becomes divided into two, of 

 which the anterior is the smallest. The 

 embryo thus becomes divided into four seg- 

 ments, of which the two foremost appear (?) 

 together to correspond to the cephalic seg- 

 ment of Argiope ; but these segments are 

 formed not, as in Chaetopoda and other truly 



segmented forms, by the addition of fresh segments between the last- 

 formed segment and the unsegmented end of the body, but by the 

 interpolation of fresh segments at the cephalic end of the body as 

 in Cestodes ; so that the hindermost segment is the oldest. Assum- 

 ing the correctness 1 of Lacaze Duthiers' observations, the mode of 

 formation of these segments appears to me to render it probable 

 that they are not identical with the segments of a Chsetopod. A 

 suspender is attached to the front end of each embryo. Before the 

 four segments are established the whole embryo is covered with 

 cilia 2 , and two and then four rudimentary eyes are developed on 

 the anterior segment of the body. 



The history of the Larva and the development of the organs of the 



Adult. 



Articulata. The observations of Kowalevsky and Morse have 

 given us a fairly complete history of the larval metamorphosis of 



1 It should be stated that it is by no means clear from Kowalevsky's figures that he 

 agrees with Lacaze Duthiers as to the succession of the segments. 



2 Kowalevsky in his figures leaves the pemiltimate lobe unciliated. 



FIG. 135. Two STAGES 



IN THE DEVELOPMENT OF 



ARGIOPE. (After Kowalev- 

 sky.) 



A. Late gastrula stage. 



B. Stage after the larva 

 has become divided into 

 three segments. 



bl. blastopore ; me. me- 

 senteron; pv. body cavity; 

 b. temporary bristles. 



