CHAP. VI. STRUCTURE OF THE HEART. 41 



Considering this uniformity presented by the heart 

 in the entire order of the Amphipoda, it cannot but 

 seem very remarkable, that in the very next order of 

 the Isopoda, we find it to be one of the most change- 

 able organs. 



In the cheliferous Isopods (Tanais) the' heart re- 

 sembles that of the Amphipoda in its elongated tubular 

 form, as well as in the number and position of the 

 fissures, but with this difference, that the two fissures 

 of each pair do not lie directly opposite each other. 



In all other Isopoda the heart is removed towards 

 the abdomen. In the wonderfully deformed parasitic 

 Isopods of the Porcellanse (Entoniscus 

 porcellandB), the spherical heart of the 

 female is confined to a short space of 

 the elongated first abdominal segment, 

 and seems to possess only a single pair 

 of fissures. In the male of Entoniscus 

 Canerorum (n. sp.), the heart (fig. 16) 



is situated in the third abdominal seg- Fig 14< 

 ment. In the Cassidinte, the heart 

 (fig. 14) is likewise short and furnished with two pairs 

 of fissures, situated in the last segment of the thorax 

 and the first segment of the abdomen. Lastly, in a 

 young Anilocra, I find the heart (fig. 15) extending 

 through the whole length of the abdomen and furnished 



amined perfectly transparent young Caprellx (probably the young of 

 Caprella attenuata, Dana, with which they occurred), but can only find 

 the usual three pairs. 



4 Fig. 14. Heart of a young Cassidina. 



