A STUDY IN MORPHOLOGY. 115 



thoracic and five abdominal somites, and terminates in a deeply-forked telson with 

 seven pairs of spines. 



This stage persists with slight change for several moults, and at the last the buds 

 for the thoracic limbs and swimmerets appear. According to GLAUS, the rudiments of 

 all the abdominal appendages can be seen at an earlier stage. 



The passage from the last of the Protozoea series to the first Schizopod stage is 

 attended by a complete change in the structure of the antennas, and these now assume 

 the adult form. The carapace also acquires two antero-lateral spines and two more 

 at the base of the rostrum. At this time it is much like Lucifer, as shown in column 4 

 of Table VI., but the endopodites of the third pair of maxillipeds and of the pereiopods 

 are rudimentary, and shorter than the very long-jointed exopodites. 



The significance of the various stages in the metamorphosis of the higher Crustacea 

 is one of the most interesting questions in the whole field of morphological science, 

 and it has given rise to at least its due share of speculation, but it will not be out of 

 place to examine the relation between the facts which have been described and the 

 various theoretical views which have been expressed upon the subject. In the case 

 of the Sergestida? it is obvious, in the first place, that the adult Lucifer and Acetes 

 also, if Acetes be an adult, are little more than mature representations of the Masti- 

 gopits stage, complicated in the case of Lucifer by the formation of a neck, and in the 

 case of Acetes by the presence of gills, and chela) on the pereiopods. There can also 

 be little doubt that the Schizopod stage of development in the Sergestidse and Pcnceus 

 bears a similar relation to the adult Schizopods, especially to Amphion, the adult 

 character of which seems to be established by WiLLEMOES-SuHM's observations (Proc. 

 Roy. Soc., Dec. 9, 1875). 



The significance of the Zoea stage in the higher Decapods is one of the most vexed 

 points in Crustacean morphology. We have shown that in the Sergestidse and in 

 Penceus the so-called Zoea stage is nothing but a preparation in the Protozoea for the 

 next or Schizopod stage ; that it involves no changes of structure except those which 

 are related to the form which it is to assume after the next moult, and that the Zoea, 

 as a distinct stage, is absent. The life-history of these forms would therefore lead us 

 to suspect that the Brachyuran Zoea is a secondary modification of the more primitive 

 Protozoea, and we may perhaps see in the larval skin which many Crdb-Zoi : as shed 

 soon after or even before they leave the egg, and which usually has a conspicuously 

 forked and very spiny telson a remnant of the unmodified Protozoea stage. 



DOHRN (' Geschichte des Krebsstammes, Jenaische Zeitschr.,' 1871) and FRITZ 

 MULLER ('Ftir Darwin') have held that the typical Zoea, with segmented abdomen 

 and suppressed thorax, is the ontogenetic recapitulation of an ancestral form which 

 has formerly existed as an adult, and DOHRN even goes so far as to recognise the still 

 more remote ancestor of this Zoea type in an embryo (" Untersuchungen tiber Ban 

 und Entwickelung der Arthropoden; eine neue Nauplius-form : Archizcea gigas," Zeit. 

 f. Wiss. Zool., xx., 597), which WlLLEMOES-SuHM has recently shown ("On the 



