PACKARD.] MORPHOLOGY OF PHYLLOPODA. Wes) 
This view as to the homologies of the limbs is directly opposed to 
what we have previously held, and to the views of Claparede* and 
Zenker; but progress in the embryology of Arthropods and of worms 
has now given us a basis for better grounded views as to the homol- 
ogy of the limbs of the leading groups of Arthropods. It now ap- 
pears that in the higher worms the mouth is, as a rule, situated in 
the first segment, the invagination of the ectoderm forming the stomo- 
deeum or primitive gullet. This is seen in the Nematelminthes (Cucul- 
lanus) in Nephelis? and in Lumbricus, and is probably common to all 
worms; and in the Annelida the mouth does not shift; it is a fixed 
point, and the first pair of tentacles arise from the first segment. So 
also is the vent or anus (proctodzum), which is the result of an invagin- 
ation of a portion of what becomes the terminal segment of the body. In 
Arthropods the anus remains invariably, in all proctuchous forms, a 
fixed point. On the other hand the mouth shifts from a position orig- 
inally in the embryo in front of all the appendages in the head to a point 
posterior to the antennee of both pairs, when two pairs are present, as 
in the Crustacea; 7. ¢., to a position in adult life between the mandibles. 
So far as we are aware we were the first to call attention to this tact 
of the change from an anterior to a posterior position of the mouth in 
relation to the antennz in our account of the embryology of Limulus 
polyphemus,? where the mouth at the time of the appearance of the limbs 
is anterior to the first pair of appendages. This was probably the case 
with all the extinct Merostomata and’ Trilobita. In the normal Crus- 
tacea Bobretsky* has shown that in Oniscus the mouth-opening is at 
the extreme end of the body, in the antennal segment, the middle of 
the procephalic lobes or antennal segment forming the front wall or roof 
of the stomodeum. In the nauplian stage in embryo of Astacus, Reich- 
enbach® has shown that the mouth is placed directly between the first 
antenne; and in the active freshly-hatched nauplius of the Copepoda, 
as well as of all the Phyllopoda, the mouth opens between the first pair 
of limbs, which become finally the first pair of antenne of the adult. 
In Peripatus Moseley has shown that the mouth opens in the autennal 
segment, which really forms the procephalic lobes. In the Arachnida, 
according to Claparéde, all the appendages are in the embryo postoral. 
In the Hexapodous insects Kowalevsky has clearly shown that the 
mouth is at first situated between the antennz, which arise from the 
procephalic lobes; before hatching it retires to an intermandibular posi- 
tion. 
The embryology of all the Arthropodan subclasses (the Myriopods 
probably not excepted, this point not being shown in Metschnikoff’s 
plates) shows, then, that the mouth is not a permanent fixed point, since 
in the embryo it is pre-appendicular, while towards or at adult life it 
assumes a position behind the antenne, when functional antenne are 
present, or in Arachnida and in Merostomata behind the first pair of 
appendages. 
An examination of the structure and homologies of the Arthropodan 
brain or supra-cesophageal ganglia shows that in the Phyllopods the 
* Claparéde, Recherches sur Evolution des Araignées, 1862, pp. 77-87. 
1Biitschli. ““Entwickhngsgeschichte der Cucullanus elegans,” Zeit. t. wiss. Zool. 
xxvi, 1876. 
20, 0. Whitman, Embryology of Clepsine, Quart. Journ. Micros. Se. xviii, 1878. 
The Development of Limulus polyphemus. Mem. Bost. Soc. Nat. Hist. i, March, 
1872. 
4N. Bobretsky. Zur Embryologie des Oniscus murarius. Zeit. fiir wissen. Zoologie 
xxiv, 1874. See Taf. xxii, figs. 20, 23. 
5S. H. Reichenbach. Die Embryoanlage und erste Entwicklung der Fluskrebses. 
Zeit. fiir wissen. Zoologie, xxix, 1877. See Taf. x, fig. 8. 
