114 PATTEN AND REDENBAUGH. [Vol. XVI. 



thoracic muscles and the longitudinal abdominal muscles. In 

 the abdominal region this membrane is double, the two portions 

 partially investing the branchio-thoracic muscles on the one 

 hand, and the longitudinal abdominal muscles upon the other. 

 Between the successive appendages it is attached to the integu- 

 ment. The space between the two portions is the ventral col- 

 lecting venous sinus (PI. IX, Fig. 6, v.c.s), which, with its 

 fellow upon the opposite side of the body, carries the blood to the 

 gills to be aerated. In the thoracic region the membrane does 

 not enclose a venous sinus. It is single and attached to the sides 

 of the endocranium, and, in places, to the integument between 

 the plastro-coxal muscles. 



The caudal spine is moved by various muscles which arise 

 by a large number of slips, each of which Lankester has desig- 

 nated by a special name. For our purpose, however, it will be 

 better to divide them into three groups and consider each 

 group as one pair of muscles, thus making one pair of flexors 

 and two pairs of extensors. The pair of flexors (PI. VI, Fig. i, 

 i.f.ni.) arise by numerous slips from the neural portion of the 

 abdominal carapace posterior to the appendages, and from the 

 outer and posterior sides of the last three pairs of entapophyses, 

 and are inserted upon the outer neural portions of the arthroi- 

 dal membrane which attaches the caudal spine to the abdomen. 



One pair of extensors (PI. IX, Fig. 5, t.e.ni.^) arise by numer- 

 ous slips from the haemal side of the abdomen posterior to the 

 heart, and from the inner sides of the last three pairs of enta- 

 pophyses, and are inserted upon the haemal portion of the 

 above-mentioned arthroidal membrane, near the median line. 

 The second pair of extensors (Pis. VI and IX, Figs, i and 5, 

 t.e.m.^) arise upon both the haemal and neural sides of the cara- 

 pace, and are inserted upon the arthroidaj membrane, external 

 to the first pair of extensors. 



Coordinate action of the muscles on either side of the median 

 line moves the caudal spine in a lateral direction. 



A sphincter mMsc\Q (Pis. VI and VIII, Figs, i, 3 and 4, s.a.) 

 closes the anus. A pair of slender muscles (Pi. VI, Fig. i, o.a.), 

 which may be called the occludor ani, arising on the neural side 

 of the carapace, just posterior to the last appendage, are inserted 



