A STUDY m 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 Claus, 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 antennae, 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 Sergestidse 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- 
gopus 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 chelae on the pereiopods. There can also 
be little doubt that the Schizopod stage of development in the Sergestidae and Penceus 
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 Sergestidae 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 Crab-Zoeas 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. 
Dohbn (‘ Geschichte des Krebsstammes, Jenaische Zeitschr.,’ 1871) and Fritz 
Muller (‘ Fur 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 liber Bau 
und Entwickelung der Arthropoden; eine neue Nauplius-form: Archizoea gigas,” Zeit. 
f. Wiss. Zool., xx., 597), which Willemoes-Suhm has recently shown (“On the 
Q 2 
