422 INVERTEBRATE MORPEOLOOT. 



the Metazoea becomes transformed into a well-marked form, 

 the Megalopa (Fig. 194), so called from the usually large size 

 of the cephalothorax. It resembles closely a Macruran, 

 differing only in the abdomen being relatively small, and 

 becomes converted into the adult form by the doubling of 

 the abdomen beneath the thorax. A Megalopa stage occurs 

 also in the Hermit-crabs, but is not so well marked off 

 from the young fully-formed animals as in the Brachyura. 



Affinities of the Crustacea. — The relationships of the higher groups of 

 the Malacostraca to one another are clearly shown by their larval forms, 

 the Megalopa showing the origin of the Brachyura from Macruran forms, 

 and the Mysis stage that of the latter from Schizopod ancestors. When 

 attempts are made to go still further difficulties stand in the way. As 

 regards the Stomatopoda it is to be noted that they pass through a sta,ge, 

 the Erichthus, in which the thoracic appendages which are present are 

 biramous, and it seems probable that both they and the Cumacea are re- 

 ferable back to Schizopod ancestors. The Arthrostraca, on the other hand, 

 are probably traceable to Cumacealike ancestors, while the Leptostraca 

 represent more nearly the Entomostracan ancestors than any other group, 

 though widely differentiated from them in certain particulars. It is even 

 still more difficult to trace out relationships of the various Entomostracan 

 orders, but it seems fairly clear that Phyllopodan' forms such as Apus are 

 to be considered as representing more nearly than any others the primitive 

 Crustacea. 



As regards the affinities with other groups very interesting questions 

 arise, two possibilities seeming to be open. According to one the Crustacea 

 have been derived directly from segmented Annelids, through forms repre- 

 sented in a modified condition to-day by Apus. The lobed appendage of 

 Apus is a modified parapodium, and the segmentation of the body has been 

 inherited. What then as to the Nauplius ? According to this view it has 

 practically no ancestral significance, or at best can be considered only as 

 representing a Trochophore larva highly modified and with many adult 

 characters thrown back upon it. This latter idea does not seem, however, 

 to agree with the facts, since the Trochophore is an unsegmented structure 

 and can be comparable only to the prostomial and first appendage-bearing 

 segments of the Nauplius. In other words, the Nauplius is comparable, 

 if comparable at all, to a Trochophore p/ns certain additional segments. 

 It has recently been suggested that possibly the Nauplius may represent 

 not the Trochophore but the larval Annelid with three parapodia, which as 

 indicated (p. 315), is a well-marked stage in the development of many 

 Polychaeta. The number of segments is apparently similar in the two 

 forms, and the idea is plausible. If, however, ia all Crustacea a gano-lion 

 representing a segment, intervenes between the archicerebral ganglia and 

 the antennulary (see p. 378), then the Nauplius has potentially one sag- 



