35 



seen, immediately dorsally to the base of the first arm (7v'. ij. The dorsal horn of the right 

 collar-cavity {6. cr a.) projects into the first body-cavit)-, the septum '/.■ in lli'^ region affording 

 origin to radiating muscles which traverse d. c} to reach the anterior wall of the proboscis. 



Fig. 146 cuts the animal at a level where the collar is just losing its connexion with 

 the proboscis. The first arm [R. i) is becoming more distinct, and the food-groove of R. 2 is 

 obvious The second and third arms are indicated dorsally by small triangular ridges. It should 

 be specially noticed that there is no trace of the food-grooves of the arms 3 — 5 ; and the direct 

 continuity of the food-grooves with the mouth, described by Masterman, appears thereby negatived. 



In the succeeding sections (figs. 145 — 141) the arms become progressively more distinct, 

 their food-grooves appearing as the series of sections is traced outwards, and their angular 

 dorsal surfaces becoming increasingly prominent. The lateral lobe of the operculum comes to 

 an end in fig. 142, and in the last section shewn (fig. 141) it is no longer seen, while the 

 arms have become completely separated from one another. It will further be noticed that the 

 complete separation of the arms from one another is preceded b\- the ingrowth of a ridge of 

 basement-membrane, so that the cavities of the arms become individually distinct before their 

 epidermic walls are completed. 



C. levins eni. 



The arms of this species are six in number on each side, and their general arrangement 

 resembles that found in C. dodecalophus. Their structure may be illu.strated by means of the 

 series of frontal sections shewn in PI. X, figs. 11 1 — 118. The histological preservation of the 

 specimens not being very good, the nerve-layer is not shewn e.xcept in the region of the central 

 nervous system and of the .strong commissural or "lateral" nerves to the stalk. But there is 

 no reason to suppose that it differs essentially from that of C. dodecalophus. 



Fig. 1 1 2 pa.sses through both proboscis-pores {p.p.). as well as through the pericardium 

 {per.) and the anterior horns of the collar-cavities (d. cr a.). The first three arms of the right 

 side [R. I — 3) have not yet separated from the arm-base; while on the left side, the arms were 

 lying in such a position that five of them are cut separately. 



Fig. 113, immediately behind the proboscis-pores, .shews both the first arms originating 

 from the anterior dorsal extremities of the collar. The six arms of the left side are seen, L. i 

 and L. 2 being connected with one another by their epidermis, although the section passes 

 dorsally to the level at which their body-cavities become continuous. On the right side is seen 

 the common base of the first three arms, posterior to the region where their cavities become 

 separate. The tip of the notochord {nc/i.), encircled by the posterior end of the pericardium (/^r.), 

 occurs immediately below the central nervous system {c. n. .v.). The proboscis-stalk is separating 

 from the buccal disc. 



Fig. I 1 4 — 1 1 7 are sufficiently explained by the lettering, but it will be noticed that in 

 passing backwards, the arms first become connected by their e]>idermis and that the complete 

 union of their cavities is preceded, as in C. dodecalophus, by the appearance of a ridge of 

 basement-membrane which may be cut in such a way as to form a complete septum across the 



