CRUSTACEA. 419 



It is beyond the scope of this work to attempt to grapple with all the 

 difficulties which arise in connection with the origin and relationships of the 

 various phyla, but I confine myself to a few suggestions arising out of the 

 developmental histories recorded above. 



Malacostraca. In attempting to reconstitute from the evidence in 

 our possession the ancestral history of the Malacostraca we may omit from 

 consideration the larval history of all those types which leave the egg in 

 nearly the adult form, and confine our attention to those types in which 

 the larval history is most completely preserved. 



There are three forms which are of special value in this respect, viz. 

 Euphausia, Penaeus and Squilla. From the history of these which has 

 already been given it appears that in the case of the Decapoda four stages 

 (Glaus) may be traced in the best preserved larval histories. 



1. A Nauplius stage with the usual Nauplius characters. 



2. A Protozosea stage in which the maxillae and first pair of maxilli- 

 peds are formed behind the Nauplius appendages ; but in which the 

 tail is still unsegmented. This stage is comparatively rarely preserved and 

 usually not very well marked. 



3. A Zosea stage the chief features of which have already been fully 

 characterised (vide p. 385). Three more or less distinct types of Zosea 

 are distinguished by Glaus, (a) That of Penseus, in which the appendages 

 lip to the third pair of maxillipeds are formed, and the thorax and abdomen 

 ai*e segmented, the former being however very short. The heart is oval, 

 with one pair of ostia. From this type the Zosea forms of the other Deca- 

 poda are believed by Glaus to be derived, (b) That of Euphausia, with 

 but one pair of maxillipeds and those short and Phyllopod-like. The 

 heart oval with one pair of ostia. (c) That of Squilla, with an elongated 

 many-chambered heart, two pairs of maxillipeds and the abdominal 

 appendages in full activity. 



4. A Mysis stage, which is only found in the macrurous Decapod 

 larvpe. 



The embryological questions requiring to be settled concern the value 

 of the above stages. Do they represent stages in the actual evolution of 

 the present types, or have their characters been secondarily acquired in 

 larval life 1 



With reference to the first stage this question has already been discussed, 

 and the conclusion arrived at, that the Nauplius does in a much modified form 

 represent an ancestral type. As to the fourth stage there can be no doubt 

 that it is also ancestral, considering that it is almost the repetition of an 

 actually existing form. 



The second stage can clearly only be regaixled as an embryonic prepara- 

 tion for the third ; and the great difficulty concerns the third stage. 



The natural view is that this stage like the others has an ancestral 

 value, and this view was originally put forward by Fritz Miiller and has 

 been argued for also by Dohrn. On the other hand the opposite side has 

 been taken by Glaus, who has dealt with the question very ably and at 

 great length, and has clearly shewn that some of Fritz Miiller's positions 

 are untenable. Though Glaus' opinion is entitled to very great weight, an 

 answer can perhaps be given to some of his objections. The view adopted 

 in this section can best be explained by setting forth the chief points which 

 Glaus urges against Fritz Miiller's view. 



27 2 



