N0.13.W. XATURAL HISTORY OF THE TSOPODA—nTC'irARDSOX. 21 



rows. Each row of cells is equivalent to a sejjjment. The mesoderm 

 cells divide more rapidly in th(^ anterior seo-ments, resulting- in the 

 formation of masses of cells on either side of the uiedian line cori'e- 

 sponding- to the limb buds, which appear on all the segments anterior 

 to the last seven. The last seven repivsent the six abdominal segments 

 and the telson, the sixth abdominal segment in the adult being fused 

 with the telson in all Isopods. 



The liml^ mesoderm and the mesoderm of the lateral masses, which 

 lies on either side of the limb mesoderm, become converted into muscle 

 and a certain amount of connective tissue. 



The liver lobes unite eventually with the posterior end of the stomo- 

 deal invagination, or stomach, and the anterior end of the proctodeal 

 invagination or intestine. The stomodeal invagination appears early 

 and comes to lie between the antennules and the antennae. As the 

 invagination deepens into the 3"olk, the posterior extremity enlarges 

 to form the stomach, the posterior extremity of the stomach uniting 

 with the liver lobes. The proctodeal invagination occurs later than 

 the stomodeal invagination. It appears first as a patch of cells lying 

 behind the teloblasts. 



The vitellophags take part in tlie formation of connective tissue, 

 muscle tissue, blood corpuscles, and perhaps even the heart. The 

 vitellophags are therefore mesoderm cells. 



The cerebral ganglia, the antennaiy ganglia, and the antennular 

 ganglia of the nauplius fuse to form the S3"ucerebrum of the adult. 



The young leave the brood pouch with the last pair of legs unde- 

 veloped. In most Isopoda there is no metamorphosis, the young 

 being similar to the adult. A transformation occurs in the famil}" 

 Gnathiida?; the 3^oung when the}^ leave the incubator}" pouch are 

 very unlike the adult males, but bear some resemblance to the female, 

 though more slender. Larval forms exist also in some of the parasitic 

 Isopods. The C3'mothoid£e have several different larval stages; the 

 body of the young is more symmetrical than that of the adult, the 

 animal apparentl}- losing its S3'mmetry on assuming a parasitic mode 

 of life. 



The early development and larval forms of the Epicaridea, a para- 

 sitic trilje Lave been most earnestl3" investigated b}' Giard and Bon- 

 nier. These authors write that the mode of segmentation in the forms 

 th(\v have studied belonging to this tril)e is holoblastic, the segmenta- 

 tion of the c<f<y Ix'ing complete and unequal, and resulting in the for- 

 mation of an epibolic gastrula." The first free larva, which the}" call 



"Prof. L. Roule also makes this statement about the egg of Asellus and Por- 

 cellio, ])ut Dr. IVIcMurrich has pointed out tliat his observations are erroneous. 

 AUhough (iiard and Bonnier have figured an eiglit-oell stage of tlie egg of Portunion, 

 in which the segmentation ai)pears to be complete and une(]ual, there may have 

 been some error of observation, and until more thorough work is done on this group 

 it is not wise to a<'<-ej(t the results so far obtained, which are so very different from 

 what has been found to be true of the of her Isopods. 



