PACiiAKD.] MORPHOLOGY OF PHYLLOPODA. O < 5 



This view as to tlie liomologies of the limbs is directly opposed to 

 what we have previously held, and to the views of Claparede* and 

 Zenker; buti)rogress 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- 

 daeum or primitive gullet. This is seen in the Kematelminthes {Cucul- 

 lanusy in Nephelis^ and in LumhricuSy and is probably common to all 

 worms; and in the Annelida the moutli 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 (proctodseum), 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 j)oint. 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 antennae of both pairs, when two pairs are present, as 

 in the Crustacea; i. e., 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 antennae in our account of the embryology of Limulus 

 polyplwmus,^ 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 antenna! segment, the middle of 

 the procephalic lobes or autennal segment forming the front wall or roof 

 of the stomodaeum. In the nauplian stage in embryo of Astacus, Reich- 

 enbach^ has shown that the moutli is placed directly between the first 

 antennae ; and in the active freshly-batched nauplius of the Copepoda, 

 as well as of all the Phylloj)oda, the mouth opens between the first pair 

 of limbs, which become finally the first pair of antennae of the adult. 

 In Peripatus Moseley has shown that the mouth opens in the autennal 

 segment, whicb really forms the procephalic lobes. In the Arachnida, 

 according to Claparede, 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 antennae, 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 

 Xdates) shows, then, that the mouth is not a permanent fixed point, since 

 in the embryo it is pre-appeudicular, while towards or at adult life ic 

 assumes a position behind the antennae, when functional antennae 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-oesophageal ganglia shows that in the Phyllopods the 



* Claparede, Reclierches eur I'E volution des Araign^es, 1862, pp. 77-87. 



'Biitschli. "Entwickkiugsgescliichte der Cucullaniis elegans," Zeit. f. wiss. Zool. 

 sxvi, 1876. 



'C O. Whitman, Embryology of Clepsine, Quart. Journ. Micros. Sc. xviii, 1878. 



3 The Development of Limulus polyphemus. Mem. Bost. Soc. Nat. Hist, i, March, 

 1872. 



■•N. Bohretsky. Zur Emhryologie des Oniscus murarkis. Zeit. filr 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. 



