MONOGRAPH OF THE EXISTING CRINOIDS. 461 



what higher in the middle section, somewhat smaller in the terminal. During 

 the free-swimming larval period it undergoes no important changes. 



With the closing off of the vestibular invagination all of the internal organs 

 are displaced toward the dorsal side, and at the same time the distance of the pore 

 from the median plane has reached about one-fifth of the circumference of the 

 larva. 



As a consequence of the changes in the hydroccele as just described the pore 

 no longer lies at the level of the region between the first and second primary 

 evaginations. The ascending branch shows a tube of considerable length which 

 rises more steeply anteriorly than was earlier the case. The hindmost end, ter- 

 minally broadened out ampullalike, lies embedded in the ectodermal body wall. 



Especially extensive is the transverse portion; on all median longitudinal 

 sections it is met with cut across as a thin-walled structure with a wide lumen, 

 rounded or somewhat flattened dorsally, placed before the gut and hydrocoele 

 ventral to the former and directly over the latter. 



In the old larvae the condition of the anterior process varies. In some cases 

 it can be demonstrated, but in most it has entirely disappeared, apparently having 

 been withdrawn into the transverse branch. There is no evidence that it has 

 degenerated and that its constituent cells have become mesenchyme cells, as has 

 been supposed. 



The walls of the canal, except in the hindmost section, have become thinner, 

 most markedly in the transverse sector, which for the greater part is lined 

 with pavement epithelium, though toward the dorsal side where it borders on the 

 right coalome the cells are somewhat higher and richer in plasma. Seeliger be- 

 lieves that the pore and the outermost end piece are ciliated. 



Meatncliyme. 



Most of the mesenchyme cells which are not occupied in the formation of 

 the calcareous skeletal elements are now assuming the form of connective tissue 

 cells, while some are transforming into muscle cells. 



The connective tissue cells occur in all possible forms in varying sizes. In 

 the youngest larvae they are predominently rounded or provided with small 

 processes posteriorly and along the major axis anteriorly, while peripherally 

 star-shaped and spindle-shaped cells are common. In older larvae, generallv 

 speaking, the size of the mesenchyme cells decreases somewhat. 



Apparently in the living animal the cells are able, through amoaboid move- 

 ments, to alter their form quickly and in the most diverse ways. 



In the lateral portions of the anterior section the processes of the star-shaped 

 cells become progressively more and more extended and begin to unite with 

 each other, so that there appears in sections, cut at a. later stage through the column, 

 a very loose connective tissue layer made up of branched fibers under the ectoderm, 

 while toward the middle rounded cells, placed much closer together, form the 

 calcareous elements of the column. On the ventral side in the median plane these 



