PRACTICAL DIRECTIONS 



373 



carapace, extending forwards laterally'and forming tire boundary between 

 the head (prostomium + 4 metameres) and the thorax (8 metameres) ; 



b, the two longitudinal branchiocai-diac grooves on the tergal region 

 of the thoracic portion of the carapace, about iinch apart : the part of 

 the exoskeleton between these covers the heart, and the part below each 

 groove forms a large plate, the gill-cover, at the side of the thorax ; 



c, the rostrum, movable eye-stalks, and epistoma (p. 349). 



7. Note the following apertures : Median — a, the mouth, on the 

 ventral surface of the head, between the jaws ; b, the anus, on the 

 ventral side of the telson : Paired — c, the auditory aperture, on the 

 dorsal side of the basal joint of the smaller feeler or antennnle (this 

 will l)e seen better later on) ; d, the renal aperture, on a conical ventral 

 elevation of the basal joint of the larger feeler (antenna) ; e, the genital 

 aperture, in the male on the basal joint of the last thoracic leg, and in 

 the female on the last thoracic leg but two. 



B. Respiratory Organs. 



1. Carefully cut away the left gill-cover with scissors, and fix the 

 animal under water on its right side, so as to expose the left gill- 

 chamber containing the feathery-looking gills. The inner wall of the 

 chamber is formed by the |3roper wall of the thorax, and the chamber is 

 open behind and below. In front of the gills is a groove, in «'hich a 

 flattened plate (see p. 353) works backwards and forwards during life, 

 driving the water out in front, and causing the bubbles already noticed. 



2. The gills are 18 in number, and each has the form of a bottle- 

 brush. The six podobranchs are external to the arthrobranchs and 

 pleurobranchs (p. 361), and each is attached to a large folded and 

 corrugated epipodite (p. 352). The gills are related to definite metameres, 

 as will be seen from the following table, in which ep stands for epipodite, 

 and V for the vestige of a gill. Note that the first pair of thoracic 

 limbs bears a simple large epipodite only. 



