PODOPHTHALMIA. 



43 



from the Lower Silurian to the Carboniferous, as representing on a, grand scale (Dithyro- 

 caris being over a foot in length) the Nebalia, half an inch in length, of the present 

 day. 



ORDER II. - - SCHIZOPODA. 



The name of this order means cleft feet, and was given in allusion to the biramous 

 or two-branched character of the thoracic appendages of the adult, both exopodite and 

 endopodite being present and well developed. This schizopodal character of the 

 appendages, which remains as a permanent feature in this group, is found only as a 

 transitory feature among the larvae of most of the Decapod a, thus clearly showing the 

 higher position of the latter order. The number of pairs of these functionally thoracic 



feet varies from six in My sis 

 to eight in Thysanopoda, 

 the increase in number over 

 that to be noticed in the 

 Decapoda being produced 

 by the transfer of mouth- 

 parts to the locomotive 

 series. The abdomen is 

 relatively very large, being 

 frequently mere than twice 

 the length of the cephalo- 



FiG. 56. My sis stenolepis, opossum shrimp, enlarged. thorax. Gills, when pres- 



ent, are attached to the 



thoracic feet, but instead of being enclosed in a gill-chamber hang freely in the water. 

 In a few forms it has been observed that the posterior extremity of the digestive tract 

 plays a part in respiration, it being richly supplied with blood-vessels, while water is 

 drawn into and then forced out from the anus. It may be interesting in this connec- 

 tion to note that a somewhat similar feature exists in some fishes. Cobitis fossilis and 

 several Brazilian forms swallow small bubbles of air, which, in passing through the 

 intestine, aerate the blood, while similar habits among the Invertebrata, - - insects, 

 worms, and holothurians, are numerous. Several species of Mysis and allied genera 

 are common on our eastern coasts, forming a prominent portion of the food of many of 

 our shore fishes. We figure Mysis stenolepis. 



In their development two types are exhibited by the Schizopoda, that furnished by 

 Mysis and that observed in Euphausia. Euphausia leaves the egg as a true nauplius 

 with its three pairs of appendages, a 

 mouth being present, though the ali- 

 mentary tract is not open at the pos- 

 terior end. With the succeeding 

 moults new appendages are formed 

 and the carapax outlined, while the 

 abdomen does not make its appear- 

 ance, except in a very rudimentary 

 condition, until six appendages are 

 outlined. A modified zoeal condition 

 now ensues, from which the adult condition is gradually produced by a series of 

 successive moults. In Mysis the eggs and young are carried in an incubatory 



FIG. 57. Later nauplius of Euphausia, enlarged. 

 I, II, VI. Appendages. 



