144 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



colour, after which they were transferred to clean sea-water till 

 the colour faded somewhat owing to the action of the excretory 

 tissue. Sections cut at this stage showed the presence of carmine 

 in the cells of (1) the atrial epithelium, more particularly that 

 part of it covering the outside of the secondary gill-arches and 

 the atrial floor, (2) a series of small tubules, previously over- 

 looked, opening (?) from the dorsal coelomic canals into the atrial 

 cavity at the tops of the primary gill-arches. These tubules, and 

 part of the atrial epithelium have probably, therefore, an excretory 

 function. 



Besides this, a pair of atrio-codomic funnels (pigmented canals) 

 occur in the 27th segment, which are possibly of similar nature. 

 Each of them is a short funnel-shaped tube, situated in the dorsal 

 coelomic canal of its side (Fig. 39), opening by its wide posterior 

 end into the atrial cavity and (I) by its narrow front end into the 

 ccelom. Owing to observation being rendered difficult by the 

 abundant pigment naturally found in the walls of these tubes, 

 the carmine method just described gave no positive results in 

 this case. 



7. Reproductive Organs (Fig. 39). Although the sexes are 

 separate there are no distinctive external characters, and the 

 reproductive organs are of the simplest possible kind, consisting 

 of 26 pairs of gonads, spermaries (testes) or ovaries, as the case 

 may be, imbedded in the outer wall of the atrial cavity. Those 

 of one side are not exactly opposite those of the other side. The 

 gonads are squarish sacs, each of which is surrounded by a special 

 section of the ccelom. The ova are just visible to the naked eye, 

 and when mature are dehisced into the atrial cavity, out of which 

 they are carried by the respiratory current flowing through the 

 atriopore. The very much smaller sperms (spermatozoa), which 

 reach the outside of the body in a similar manner, are tadpole- 

 shaped, each of them possessing a short conical head and a 

 vibratile tail. 



8. The muscular system (Figs. 38 and 39) exhibits well-marked 

 segmentation, for the great lateral mass of muscle on each side is 

 divided into a series of (in A. lanceolatus) 61 or 62 > shaped 

 muscle-segments (myomeres, myotomes), separated by connective- 

 tissue septa. Those on opposite sides do not correspond. The 

 constituent muscle-fibres run longitudinally. Swimming move- 

 ments, consisting in bending the body first to one side and then 



