166 CRUSTACEA. 



procerebium) gives off the nerves to the eyes and to the frontal and other sensory 

 organs, the second section the nerves to the first pair of antennae (na f ), while 

 the posterior section (c 3 ), which lies on the course of the oesophageal commissure 

 (sc) behind the oesophagus, supplies nerves to the second antenna (na"}. Among 

 other Crustacea, the antennal ganglion undergoes more or less displacement 

 forwards along the commissure and a subsequent fusion with the brain. The 

 acceptance of this view of the alterations in position is attended with a certain 

 amount of difficulty owing to the course of the transverse commissures (origin- 

 ally running behind the oesophagus) connecting this pair of ganglia. The following 

 alternative is offered us : either to suppose that a secondary pre-oral transverse 

 connection is developed, or to assume that the transverse fibres, after the ganglia 

 have completed their wanderings, retain their original course behind the oeso- 

 phagus. GLAUS (No. 78) believes that this primary connective retains its original 

 position in the adult, and he would identify as that structure the transverse 

 post-oral connection between the oesophageal commissures, which is found in 

 many Crustacea (Apseudes, Stomatopoda, Decapoda), lying in front of the 

 mandibular ganglion. In other cases, this fibrous connective is perhaps fused 

 with the transverse commissure of the mandibular ganglion. 



The question now arises whether we are justified in considering the part 

 of the brain lying in front of the antennal ganglion as an originally single 

 complex, or whether, in this also, a separation into (two) consecutive segments 

 can be recognised. We must here mention KAY LANKESTER'S theory (No. 15), 

 which distinguished in the Crustacean brain an anterior section connected with 

 the optic ganglion under the name of archicerebrum, this only being enlarged 

 into a syncerebrum when the ganglia of two following segments (the antennular 

 and antennal segments) are drawn into it. This view has been accepted by 

 PACKARD (No. 86), who distinguishes in the brain of Asellus as separate 

 sections : (1) the optic ganglia, (2) the procerebrum, (3) the antennular ganglia, 

 (4) the antennal ganglia. In this case, we should have to find the homologue 

 of the Annelidan brain, which develops out of the neural plate, in the pro- 

 cerebrum alone, while the optic ganglia would be a secondary portion of the 

 brain* arising during the later development of the paired lateral eyes, and 

 the antennular and antennal ganglia would be ganglia of the ventral cord. 

 This view stands opposed to that of GLAUS (No. 78), according to which the 

 antennular ganglia, together with the procerebrum, formed an originally single 

 complex, the primitive brain. This portion, which is to be deduced from the 

 neural plate of the Annelidan larva, contains the ganglia of the former median 

 sensory organs (Nauplius eye, frontal organ) and the anterior antennae, which 

 are morphologically to be homologised with the Annelidan palps already con- 

 nected with the neural plate. This latter view would be supported by the 

 observations of REICHENBACH, according to which the rudiment of the corre- 

 sponding part of the brain is originally to be found as a single complex at 

 the bases of the first antennae, and only later breaks up into two pairs of 

 ganglia. A certain amount of support is also afforded by the peculiar structure 

 of the first antennae, which, as carriers of important sensory organs, do not 

 develop in accordance with the fundamental type of the Crustacean limb, a 

 point specially emphasised by CLAUS and BOAS. Such a heteromorphous 

 structure of the an tennule might, indeed, be secondarily acquired, and might 

 be accounted for by its physiological significance already mentioned, as well as 



* A view first enunciated by HATSCHEK (Beitrage zur Entwickelungsgeschichte 

 der Lepidopteren), and later accepted by GROBBEN for the Crustacea. 



