viii PRACTICAL DIRECTIONS 411 



lo the anus, and giving off near its anlerinr end the Jiepatic ciciiiut 

 or liver, extending forward on the right side of the pharynx. 



8. The inyoiiieres and the iniermuscular septa lietween them, arranged 

 like a series of V's, the apices of which point forwards. The muscular 

 fibres run longitudinally from septum to septum. 



9. The gonads (ovaries or sperinaries), arranged in a .single row on 

 either side of and rather further back than the pharynx, extending as 

 far as the atriopore. Note in the female the large ova, which, when 

 ripe, cause a great distension of the body ; and in the male the minute 

 sperm-cells, the structure of which cannot be made out in entire 

 specimens. Sketch. 



II. Cut a specimen into short pieces, about an eighth of an inch in 

 length, and select portions from — a, between the hinder end of the 

 atrium and the anus (Fig. 100, B) or through the latter; b, just in 

 front of or through the atriopore ; c, through the anterior part of the 

 pharynx ; and d; through the posterior part of the pharynx (Fig. 100, 

 A.). Stain and imbed these, and prepare a few sections from each 

 (p. 136). Examine in the order given above, first with the low, and 

 then with the high power. Sketch a section from each region. 



a. r. The oval form of the section, the median dorsal and ventral 

 fins, and the integument (columnar epiderm and thin derm). 



2. The myomeres cut across in various planes, and appearing as 

 squarish masses separated by the septa. The muscle-fibres are mostly 

 striated. 



3. The central nolochord, oval in transverse section, with transverse 

 wavy lines indicating the boundaries of the vacuolated notochordal 

 cells. It is surrounded by a connective tissue sheath, continuous with 

 the connective tissue investment of the neural canal above, and with 

 the intermuscular septa at the sides : the latter pass into the derm 

 peripherally. 



4. "[\\t spinal coi-J, lying in the neural canal. It is ovoid in trans- 

 verse section, and has a dorsal fissitre (see p. 155) extending downwards 

 nearly to tfie central canal, which is nearer the ventral than the dorsal 

 surface. See if your section happens to pass through a dorsal or a 

 ventral nei-ve. The dorsal nerves arise by large single roots from 

 about the middle of the sides of the cord, while the ventral nerves have 

 numerous fine roots, and arise from the ventro-lateral angles of the 

 cord. The dorsal and ventral nerves are not in the same transverse 



lane (compare p. 163), liul have an alternating arrangement. 



